Large lump under bug bite? Allergic reaction to bite?

I got bit by something, not sure what it was because I was sleeping, I woke up with it (I know what bug bites look like), it looked like a normal bite from a mosquito. I was scratching it, and I realized there was a large lump around it. All you can see is the red bite, but if you would run your hand over it, you can feel the big lump surrounding the bite. Is this just an allergic reaction to the bite? Give me all info and experiences you have! Thanks so much!