you can get a dna test without telling your dh.. they have them online and you swab and send them in and the get the results later... test and give yourself peace of mind if you want but I say let sleeping dogs lie... let your dh be the father.. if he has no doubts just let your baby have his daddy without all the drama
going by your due date its your husbands baby. not to mention you had a period after you had sex with this other guy. i say get a dna test rather than guessing but it does sound like its your husband.
If you actually had a period it wouldn't be the other persons baby.
Only a paternity test can answer that question. Nobody here can shed any light on your unfortunate situation.