Did she CHEAT or am I being STUPID!. Please Help..?

So here's the deal, me and my girl been together for 6 years and we are tell each other everything..and we love each other...well recently she went on vacation to florida with a girlfriend, and my girlfriend called me every morning, every afternoon, and every night, nd I know she didn't do anything, cuz she even told me if guys hit on her she would tel them off..but she came back and we been textin dirty pics about how much we wanna have since she's been gone for a week and stuff..so tonight we were gonna do it and she says she's not in the mood..she said she did it 2 herself, and that her back is too sunbured for her 2 concentrate on ..Is this a reasonable reason or do u think something else happened...she did give me , so am I just over reacted.I'm just confused if she missed me so much and said she couldn't wait, why would she do it to herself, knowing I'm coming over..girls please explain