Hay, I haven't personally experienced that pero a few of my pregnant friends did get lumps or swollen areas under their arms during pregnancy, usually from lymph nodes reacting to hormonal changes or sometimes just stretched skin and tissue. Since you're 35 weeks na at masakit pa rin, mas safe na i-check mo agad with your OB or midwife, sis, kasi they can properly assess kung anong exactly ang cause at ma-rule out kung may infection or kung normal lang talaga. Don't worry too much but don't ignore it din, okay?