picturing a different man durning sex. HELP!

So the other night I couldn't get off, I love my husband, and have been with him for 3 years and we have a child toghether, I love having sex with him, and have had no problems prior to this one,  I'm currently 3 months pregnant and for some reason, I have no idea why another guys face kept popping up, I'd close my eyes and I wouldn't see my husbands face but an old EX, which makes it worse! after I felt so guilty like I cheated on him mentally, and I'm still feeling super guilty, I've never cheated on him, or thought about cheating EVER, and I hate my ex ! It's been a little over 5 years since I last seen my ex, he cheated on me so many times and abused me, so I really just can't stand him, I feel like my hormones are all over the place 😰 and I feel like I'm afraid to have sex again because what if it happens again. Has anyone ever thought of  another guys face during sex by accident ! I'm seriously so confused on why it happened! I feel so alone.