Better to be angry than devastated. It will get you through. You sound very strong, backscratcher. You're doing well. Mrs Mooks' suggestion of texting him to say you're feeling better and will stay at your sister's is good.
I've been lied to many times by men (I'm a bad picker) and have learnt that they will lie to your face without batting an eyelid or betraying any nervousnous or hesitation, and will have plausible answers ready, when questioned. This happened to me.
Is there any one name that he's started to mention more often- it crops up here and there? Any new people he's come across at work, socially or in a team or group he is involved with?
Has his appearance changed - hairstyle, growing or shaving off a beard etc, Is he suddenly taking a greater interest in diet and fitness? Is he dressing better?
Has his last libido changed or is he more rubbish/selfish in bed than usual?
Working late is a classic excuse, as is taking up a new hobby or seeing much more of a RL male friend than he usually would.
I know gay guys who've had affairs with married men and the wife didn't suspect as she thought they were just friends.
Men don't have to be unhappy with you to have an affair - they'll do it just because the opportunity arises. Not all men are like this but in my experience men are naturally more polygamous than women. They don't have the same emotional investment in sex.
I'm not trying to make you miserable, back scratcher, just trying to offer some insights.
While this text may have an innocent explanation in isolation, other factors looked at together point towards the possibility of an affair.
Another tactic they will try is to instigate a row with you that then gives them an excuse to storm out and see the OW/OM under the pretext of going to see their RL friend who they've suddenly become joined at the hip with.
As PPs have suggested, search his car for clues, including under the boot / spare wheel etc. Also if he has a shed or garage, have a look round there.
Look at his viewing history on his computer - I caught an ex out this way.
Most importantly, don't confront until you have the evidence. Thinking of you and hoping that al of these "bad" clues are just an unfortunate coincidence.