Actually I think you need to tread carefully here. if you just confront him, he WILL gaslight you. Tell you it was a joke, it meant nothing, you're overreacting, you're being too sensitive. Then he'll get annoyed and blame you for being insecure, ridiculous and how dare you accuse him of doing something bad. he'll turn it round that you're the psyco having a go at him for something he hasn't done. If it was actually completely innocent and he really is that thick he will be concerned that its upset you, he would be asking you what he can do to put your mind at rest and be bloody severely apologetic.
Firstly, do you have access to his phone at all? If you can look at his phone, if there is an emotional or physical affair going on, there is likely to be some signs of that on his phone somewhere. Or in his emails or in a chat app. I think this is an occasion where snooping is totally justified.
Secondly, do you have anything at all to do with any of his work colleagues? Are you friendly with any of them? Assuming it is actually someone at work he bought the card for. Can you arrange a surprise drop in to the office for any reason? Is it that sort of place where this would be ok? If you take baby, most women (and some men) I know in offices I've worked in will come over and show an interest in the baby and be delighted you bought baby in too. You could see who seem reluctant to come over and do this. You could also see if anyone in particular seems to look slightly uncomfortable with you being there (including your DH). If they are, theres probably something going on.
At this point I think its best to perhaps keep quiet a little longer and just start taking notice of anything he might be doing out of the ordinary. later nights at office, meeting with 'friends' on evenings and weekends, being secretive with his phone. At some point, if he is potentially pursuing someone or has already done so, it will become apparent in some form or another.
It will be really hard to keep quiet, trust me I know. But I think if you bring it up now, you are essentially alerting him to the fact you suspect something is going on and he will be very careful to cover his tracks.
I honestly cannot though see any innocent reason he bought a valentines card for another woman. And like you say to have done so at the same time he bought yours is a double kick in the guts.
If you do bring it up with him though, keep calm and see what his story might be this time.