Why do I feel irritated after sitting for long hours? After sitting for 2-3 hours in a day there is a sense of annoyance that follows the littlest of things that are normally overlooked. Maybe this is because of heightened awareness or lack of tolerance for family and friends. It even happens at retreats during the earlier days. My teacher says sankharas are coming up. 

So is it right to assume that the more I sit, the more annoyed I will get and the more annoyed I will get I the more I should sit? If that makes sense. 

I do practice 5-10 minutes of metta bhavana at the end of the sit. But I don't feel like it comes naturally. And to my knowledge forced metta is as good as no metta?
These can arise if you do not develop the right type of concentration and awareness


If your meditation is an active struggle to keep your focus on a particular meditation subject stress arise 
If you are not equanimous to sensations (and reacting to them with dislike or liking) during meditation your will get stressed


If you abandon the above two meditation will reduce stress.

Kindness meditation also can help. 1st you should create peace and serenity in your mind and then wish you share this with other. If this is not there in your mind and / or you are not sincere, this will be just repeating words. So 1st examine your self to see if you have happiness within you then start kindness meditation.

Being week in morality also causes agitations and leads to manifestation of the hindrances. So you base of morality should be strong.

When past fabrications come up it gives different sensations. In this case most likely unpleasant or not being a sensation you crave for, which is causing the problem. What ever sensation or experience you get be equanimous. Then you will not get stressed.