I think he's cheating on me

I've been going out with my boyfriend for about 8 months now and we've had our ups and downs as I've been going through a chronic illness. Last week he turned up on Saturday (after going out Friday night with friends) and I noticed he had a red mark on his neck so I asked him what it was coz it looked like a hickey but he claims it was a mark from his t-shirt even though it was too high up for that and he had 2 scratches on the back of his neck but I didn't question it he seemed off with me really quiet when he was here and he went the gym apparently for 8 hours mentioning he met up with his mum but not too sure if I believe that. This week I found another profile on fb he had created with a recent pic and it said he was single, he's at the gym again now so think I'm going to ask him about it but if I'm wrong I haven't got the strength for another argument. Just wondering what I should do coz I haven't got anyone else to talk about this with.