Hi. I Noticed When I Had My Period 3 Weeks

Hi. I noticed when I had my period 3 weeks ago that I couldn't find the last tampon. I thought maybe it had just came out or something. Started getting cramps and noticed an odor. I have a UTI so decided to check out the strings and noticed there was the tampon. It was very dark brown and foul. For the past two weeks I haven't been feeling good at all. Upset stomach, nausea, tiredness, acid reflux, cramping, and bloating, etc. Not sure if this could be due to this. I have some antibiotics I'm going to try to take or should I seek getting some blood work done or any other tests?