Found new lump in L armpit 4wks ago. Firm, bean shape, smooth, slightly tender, no response to antibiotics; awaiting u/s. Am 35wks pregnant&scared.

Armpit lump. Is lump on the skin or inside your armpit? If lump located on the skin, after delivery which is about 5 weeks, it can be either excised or have dermatologist to biopsy it. If inside the armpit, after delivery it may go away by itself. It may be enlarged lymph node due to breast engorgement. Please do not worry too much. It does not sound a serious problem.
Lump. Its probably a lymph node. If you shave your armpits, it might be from that. Since you're near the end of pregnancy if needed it could be biopsied after delivery, but most lymph nodes happily are benign. Would be more concerned if rock hard and immobile.