Why have we been getting on each other nerves? Why was the sex so good?

Today is the first time in three weeks my girlfriend and I have had sex or even slept in the same bed. Not only do we work today we live together. I don't know why but lately we been getting on each other nerves a lot more than normal. So I've been sleeping on the couch of my best friend. For some reason she texted me and asked to meet for dinner. After dinner we went by home and started watching a movie. Next thing I know one thing lead to another and we was in bed together. Tbh it was the best sex we have had in a long time. How do I tell her that i I'm sorry without making it seem like it's my fault we wasn't getting along. Plus why was the sex so good
Why have we been getting on each other nerves? Why was the sex so good?
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