Could he be cheating or is it something else?

Ok so for two days now i have been asking my husband to have sex with me. His first response was he didnt want to, when i asked him why he said cause i was on my period. Im not on my period though i dont start for another two days. Then he says its cause he is tired cause he has been sleeping all day. So i told him its whatever and to let me know when he wants to have sex. To me it sounds like he is making excuses. This isnt the first time he has done this. We have been married for three years, our sex life was great in the beginning then it kind of just died down and he started doing this making excuses. When it first started i found out that he was texting other women and was on dating sites and porn sites. Then things got better, now he is doing it again. Should i be worried? Could he be cheating on me? What should i do? Please no hateful comments.