Aw mama, I feel you, super annoying talaga ang kati ng armpit rashes, lalo na pag pregnant kasi mainit ka na eh! A few of us moms experienced similar rashes during pregnancy kasi of hormonal changes, sweat, at friction especially kung malalaki na ang bump, but since hindi mo na-experience sa first pregnancy at medyo persistent pa rin despite calamine, I'd really suggest checking in with your OB just to be safe, baka may specific skin condition or fungal thing na kailangan ng different treatment para hindi lumaki. In the meantime, keep the area dry, wear breathable cotton clothes, at avoid heavy creams na makakahibot pa ng init. Hoping it clears up soon, sis!