I've had this problem with ex's and from talking to them I have had the responses "that's what you do in the beginning of the relationship" and "I wanted you to like me".

I was happy in the early days, with both the frequency and the quality of the sex because I thought I had found someone that my libido matched with.

A few months into the relationship, after I'd fallen in love and the relationship had reached the point where we could be comfortable around one another, the rejections, excuses and "fine, but make it quick" statements had started. This caused arguments and the girl blaming me for always wanting sex, using her for sex etc. 

I wasn't behaving any differently than the early days of the relationship, it was the girl that changed. "I wanted you to like me" made me realise that this was a form of manipulation, using sex to secure the relationship, then returning to their true libido level.

You should ask her why she used to have sex so often, but not anymore. There could be genuine (possibly medical) reasons, but if she gives you an answer that states that she was having so much sex because you liked it, then its possible she manipulated you.

You've not changed and have therefore done nothing wrong.

If she's changed because of a medical issue, then neither of you have done anything wrong, but she should get treatment.

If she's changed because 3-5 times a week wasn't her true libido and now she is comfortable with you has reverted to her true libido, then she is in the wrong for misleading you. 
If she has mislead you and tries to make you feel guilty for wanting a return to the 3-5 days a week then she is gaslighting you.