Yes, you need to get her in to a vet. Only a vet can diagnose and treat this.
It is important that you get her in ASAP.
In the mean time, if she is eating, drinking, peeing and pooping, then keep an eye on her and watch for the abscess to burst or for any complications.
You could add a picture here if you like. And, please let us know how the vet visit goes.
If you are still feeling unsure, you may give me a call. My number can be found in the "who to call" thread of this forum.