To answer your questions:
1) There may be some breakdown in the skin that cannot be visible to the naked eye. This can cause an entryway for bacteria to enter, and possibly infect the lipoma.
2) That is a possibiliy. Skin infections (i.e. cellulitis), or infection of the lipoma, do not necessarily need extremely broad-spectrum antibiotics. Narrower-spectrum antibiotics, such as dicloxacillin or Keflex may suffice. However, without reviewing your case, it is impossible to say for sure. You should discuss this with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_b