Thank you for asking the query:
There are few possibilities here, I will discuss it here as bullet points. Let me say this before hand – This does not look like Cancer. Though it can cause blood in urine that will not be in our first set of disease we look into – taking into consideration of your age and specific symptoms.
1.We need to ascertain where the bleeding is from. Even though you feel the bleeding is in the urine, It could be a easily be a vaginal bleeding getting mixed up with urine. This can happen in almost 50% of cases. If that is the case, if the bleeding is from the uterus, cervix or vagina then the most probable causes in your case would be a cervical polyp ( a harmless outgrowth ), infection ( even though you tested negative for STI, still) or uterine cause like endometriosis.
2. If the bleeding is from Urinary tract itself (that is pure bleeding from urine), then the causes can be an urinary tract infection or a stone in the urinary tract. There are lot other causes that can be considered, but it’s essential to approach it a stepwise manner and rule out most common ones.
3. IUD - CAN cause bleeding, particularly if it is displaced. So the IUD needs to be removed. Also note . When you have an IUD, however, bleeding after intercourse may mean that the IUD has moved from the top of the uterus to the lower part, which may render it ineffective. So you could use an alternative method for contraception.
4. Cancer and haemangioma ?Very less likely in your case.
So what’s next ?
1. I recommend a consultation with a OBGYN. A physical examination and history will easily give a diagnosis. If it’s a polyp it can be treated without much trouble. On the other hand the antibiotics will do the trick. If the bleeding is related to kidney and urinary tract you may need a referral to urologist for further evaluation.
2. Investigation - Complete blood count and urine routine studies. If the diagnosis is not evident from this you may need further tests like Xray KUB, Urine culture, CT scan etc
It is not a very serious problem so do not panic. I would recommend you to consult with a gynaecologist to further diagnose and treat this problem.
Hope this helps. Feel free to ask if you have further queries. Thank you.