Why is she so frustrated ?

So I've bee going out with my girlfriend for almost two years now. We live together and enjoy many activities and hobbies together. I'd say that in general, our relationship is doing well. The only issue would be the sex part. I'm a male and do like to have sex more than her.

At the beginning of our relationship, it was easy to have sex. She would come to me often for it, and I would do the same. Now that it's been two years, things have changed. She doesn't come to me for sex often (maybe once every two, three weeks) and if I go to her, it will frustrate her a lot of times. The sex is still really enjoyable when we do make love and I know she thinks likewise. We had discussions over this and what I know for sure is that if she's not in the mood, she just doesn't want to have sex, period. She doesn't let much space for temptation either. We have started fighting over this in the passed few months and it's worrying me. One thing that worries me most is the fact that she seems to want to be able to decide when we have sex, or when we don't. It feels really unnatural and forced.

I'm kind of out of clues as to what to do with this. I've tried giving her more space and not being so "needy" but it seems that she is still finding reasons to get mad at me about sex. I'm worrying that she is using sex to nourish some sort of frustration inside her. Frustration for what ? I'm really not sure. She could also be using this frustration for control as she knows it makes me feel really bad when she's frustrated like that.

If anyone would have any advice on this, it would be greatly appreciated. I'd simply like to have some tips as to what I could do for things to get better.

Thanks.