HCG rising after dropping to 4, post MTX

Hi, wondering if anyone experienced this and, if so, what was the resolution. I had a PUL with a mass on my left ovary, received a methotrexate shot mid June, HCG peaked around 2000, dropped steadily after. I had periods midAugust and midSeptember. BUT: HCG dropped to 4 in August, then rose to 5, and a month later, is now at 7 ?! GYN recommended expectant management, saying they repeat MTX if HCG is 200+. Please help me figure this out.