Ay sis, nakakahiya pero ganyan talaga ang lutong-bahay ng ating katawan! So based sa timeline mo, that light spotting on Jan 17-18 could be ovulation bleeding, nangyayari talaga yan sa ilang moms, usually just a drop or two lang like what you experienced. Pero considering na may cramping ka in the lower back area at acidic feeling, a few of us noticed that combo can also mean early pregnancy signs, especially kung nagawan kayo consistently ng Jan 29 onwards. The thing is, ovulation spotting at early pregnancy spotting can honestly look parang magkakaiba, so it's really hard to tell without a test or check-up.
What I'd suggest is try a pregnancy test muna in a few days kung still walang period by late January, mas accurate yan, or better yet, book a quick appointment with your OB para ma-rule out any concerns like hormonal imbalance o baka something else causing the acid feeling and back pain. Mas peace of mind lang talaga, lalo na may cramping ka na!