As you can see by these answers, you weren't clear on what happened. If would be helpful if you'd answer the questions people asked.
My interpretation is you and mom each enclosed cash or a check for $100 in the same card. The bride and groom assumed the money came from mom and bf, so mom and bf got the thank you card and you're pissed because he's getting credit for your gift.
If this is close, the mistake you made was putting your money in someone else's card. This doesn't excuse the bride and groom, because that kind of carelessness isn't ok. But there's not much you can do about it.
If it was in different cards, etiquette says it's fine to ask about, because you want to be sure it wasn't stolen. But I'm guessing that wasn't the case.
Finally, $100 is a very generous gift. If it's more than you can easily afford, why did you give that much? Technically, the only mandatory item at a wedding is a nice card. Most people do give gifts ,but it should never be more than you can afford.