AIO Found Another Girl's Underwear In Husband's Truck

I found another girl's underwear in the glove box of my husband's pickup truck. He's a carpenter and also does night jobs shoveling fish guts. Also they were pretty fancy, had the initials JS on them.

He swears he's innocent, even tried to tell me we weren't really married and I wasn't the mother of our four boys. His friend told me that they were his, or actually a phone sex girl he got lucky with and borrowed my husband's truck. He said my husband was covering for him and he does have the reputation of being a bad boy.

Should I believe my husband or am I going overboard with this?