girlfreind/fiance is always too tired for sex. or doesn't want to have sex?

I'm 19 and she is 20, we have been together going on 3 years now. Like any other relationship we have had our ups and downs, but have always fixed our problems becouse we love each other.

Well here recently (about a month now) I've started a new much better job that consists of me staying out of town, about 2 hours away, during the week (monday-Thursday) and i usually come home for the weekend friday night- Monday morning

So that narrows it down to weekends we should be spending time with each other, cuddling, and of coucourse having sex.

Well it never fails that every night we lay there cuddling and I rub on her and try to get her attention and affection, and she just goes to sleep.

I ask her in the mornings why she didn't wanna do it and she says (I was too tired)

and I'll even ask her, "will we be able to do it today"? And she answers with "i don't know"

I want to know what's going on. She used to be the one to nearly rape me cause she would always want it. Now I'm lucky to get to make love 2 times a month.

Another thing is, while I'm out of town during the week I like to call her before bed and talk to her, and gradually text her some during the day, it never fails her phone dies every night and she never puts it on a charger till she lays down and falls asleep.

I look forward to getting tk talk to her everynight, and it seems that's the last thing on her mind.

I don't know what to do:/ I know she isn't cheating, becouse our house is right next door to my mom.