Could you be more specific, please? Are they on the outside of the skin or are they deeper inside? What do they feel like? How large are they? If you can see them, what do they look like? Is the rabbit a boy or girl?
If you're sure there is a lump, it most likely needs to be checked by a vet. Possible causes of lumps in the tummy area are abscesses, bot fly larvae, and cancers, including mammary (breast) cancer which is primarily in unspayed girls (or girls who were spayed only recently). It's not a good idea to give any medication to a rabbit without a vet's recommendation unless you are very experienced with rabbit health problems.