Falling out of love??

My husband and I have been married for almost 7 years. We have 2 kids together. He has always been so nice and attentive to me, but lately I think he is changing his mind about everything. All he wants to do is be left alone. The only time he ever shows me any attention is when he wants . Sorry if thats to much info. We got married when I was 18 and he was 19. I just wonder if he feels like he missed out. Plus he is obsessed with checking out other girls and making it a point to flirt and stuff right in front of me. Is this the beginning of the end. Please no mean remarks!!