Ay, nakaka-relate ako sa situation mo! So based sa timeline mo, active kayo Dec 29 to Jan 12, then spotting ng Jan 17-18, that's roughly 3-4 weeks after, which could actually be implantation bleeding rather than ovulation spotting. Implantation usually happens around 8-10 days after conception, so the timing matches.
Ang thing is, one of my mum friends had almost the same experience, light spotting after she thought she had her period, then after a week or two she realized hindi siya regular kundi pregnant na pala. Ang sore back, the acid reflux feeling, at yung spotting combination, those are actually common early pregnancy signs din, not just ovulation. With ovulation usually it's just one day or walang symptoms, pero pregnancy symptoms tend to linger and build up.
Pero honestly, best way to know is mag-take ng pregnancy test na, especially kung ang spotting mo at symptoms continue. If it's still too early (less than 5 days after spotting), wait a few more days para mas accurate ang result. And definitely check in with your doctor once you get clarity, especially with the back pain, kasi gusto mo ma-rule out kung may iba pang cause. Good luck, momsh!