Is it haram for a married woman to have feelings for another guy without acting upon it?

Yes, it's haram. Did you think that I would say it'd be okay? No, you can't have feelings for, or you shouldn't entertain these thoughts or these feelings. Don't let shaytan come and whisper to you, particularly when you're in a marital relationship. Focus on your marital relationship. Once you're able to fix those feelings and that marital relationship, then inshallah you'll be flying and you'll be fine. But don't let shaytan come and put these whispers into you. The most beloved thing to shaytan, as we know, based on the hadith in Sahih Bukhari, is when he breaks up marital households, marital families. So don't let shaytan have this effect on you.