When should the start of my period be ???

On February 14th I started antibiotic medication and I was due for my period on the 15th. I started to just spot for like 5 days on the 15th and it wasn’t enough to use a pad . It was stringy light brown and reddish discharge. This happened from the 15-19th. The day after the 20th it was more heavier to use a pad it lasted three days . I don’t know if my period started on the 15th? Or it started the 20th because it was heavier on the 20th.I’m unsure what to think any opinions???