Could it be the transmission?

Good afternoon everyone,
I'm new here and could sure use some help. I have a 1999 Chevy Cavalier (90,000 miles), with what I think must be a transmission problem. It just started today. I pulled away from a light and there was a lag, then a thunk noise. The noise repeated as the car gained speed and shifted gears. I can hear ya'll laughing, but I don't know how else to describe it. Up until a couple of years ago, I would have had my husband take care of it, but he passed away and I'm left with car repairs on my own. I know my little car is old, but I have to keep her running, she's my only transportation. I've had very little trouble so far with her. Any help would be appreciated very much.
akvale