If fasting causes heavy menstrual bleeding, anaemia and lethargy, can she not fast?
Question

I hope my message reaches you in good health and emaan. I have a complicated situation regarding menstrual bleeding and fasting.
In the past 6 years I have suffered irregular menstrual cycles. I have noticed that every Ramadan or when I fast I tend to get prolonged heavy bleeding, that last for more than 10 sometimes 2 weeks.
Alhamdulillah by the will of Allah my husband and I went to hajj 1440/2019 and since then my period has been regular, every 28 days, 5-7 days periods, no clots and what is considered as a period. 8 months of a good healthy cycle since hajj, It is now Ramadan 1441/2020 I had fasted some days during the month of rajab and shabaan 6 days before ramadan I started bleeding 4 days before my menses was due. For 11 days I had brown discharge before getting a red flow.
Should I continue fasting knowing that fasting could be a pattern of prolonged bleeding, irregular bleeding, a cause for poor health such as anemia, light headedness and lethargy?
I don't know what to do other than rely on Allah the one whom gives shifa and removes all worries and tribulations but this matter is causing me much distress.
Praise be to Allah.Firstly:
If fasting will cause you to bleed heavily, and that will result in clear physical harm such as anaemia, then you should treat this problem and make up for the low levels of haemoglobin.
If your situation does not improve with treatment, then this is a valid excuse which makes it permissible for you not to fast.
Then you should look at your situation after that. If there is the hope of recovery, then you should make up the days that you did not fast later on, because Allah, may He be exalted, says (interpretation of the meaning):
{So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]} [al-Baqarah 2:184].
If there is no hope of your recovery or becoming able to fast without harm, then you do not have to make up those days; rather you only have to feed one poor person for each day, because Allah, may He be exalted, says (interpretation of the meaning):
{And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day]} [al-Baqarah 2:184].
Al-Bukhaari (4505) narrated from ‘Ata’ that he heard Ibn ‘Abbaas reciting the words: {And upon those who are able [to fast, but with hardship]} – that is, those who want to fast but find it hard – {a ransom [as substitute] of feeding a poor person [each day]}.
Ibn ‘Abbaas said: That was not abrogated. It refers to the old man and the old woman who are not able to fast, so for each day they should feed one poor person.
By analogy, this also applies to the sick person for whom there is no hope of recovery.
It says in Fataawa al-Lajnah ad-Daa’imah (10/198): When the doctors decide that there is no hope of recovery from this sickness from which you are suffering and which makes you unable to fast, then you must feed one poor person for each day, giving half a saa‘ of the local staple food, whether it is dates or anything else, for past and future months. If you give a poor person dinner or lunch for the number of days that you owe, that will suffice. As for giving cash [instead of food], that is not acceptable. End quote.
What we advise you to do is consult a female doctor and raise your blood haemoglobin levels through medication. In the future, you should prepare for Ramadan by taking medicine before the month begins.
Secondly:
With regard to lethargy, lightheadedness and mild headaches, these do not make it permissible not to fast. These are things that usually happen to one who is not used to fasting.
Ibn Qudaamah (may Allah have mercy on him) said: The kind of sickness that makes it permissible not to fast is severe sickness that is exacerbated by fasting, or it is feared that fasting will slow down recovery." (Al-Mughni  3/155).
It says in Kashshaaf al-Qinaa‘ (2/310): A sick person who is not harmed by fasting should not break the fast, such as one who has scabies, toothache, pain in his finger, boils and the like.
It was said to Ahmad: When can a sick person not fast? He said: When he is unable to fast.
It was said: Like fever? He said: What sickness is worse than fever? End quote.
So seek the help of Allah, may He be exalted, put your trust in Him and seek the reward of fasting, for the one who fasts Ramadan out of faith and in the hope of reward will be forgiven his previous sins. Narrated by al-Bukhaari (1910) and Muslim (760).
And Allah knows best.