Hi, and thanks for your medical question on eHealth's Ask a Doctor forum.
Since you are having negative home pregnancy tests even 2-3 months later, there is hardly a chance you can be pregnant. However to confirm please go for an abdominal ultrasound. Apart from detecting pregnancy it can also detect cause of bleeding after sex such as uterine polyps, endometriosis, and uterine fibroids.
The other causes of abdominal pain and bloating, and missed period is polycstic disease of the ovary. Pelvic inflammatory disease can also be a cause for this. This can also be detected by ultrasound. There are many other causes for irregular periods such as high cholesterol, hormonal imbalance, highly stressful life, chain smoking, and dramatic changes in life style. These should also be looked into.
Discuss these possibilities with your doctor. It is difficult to comment beyond this without examining. A comprehensive investigation is required keeping all the points in mind. Hope this helps. Take care!