Fortunately "_ _ played around with bf first time, he ejaculated in back._ _ " this has nothing to do with the problem you described.
Fortunately your problem is also not because of underweight
Pain could be related to gastrointestinal system. You family doctor will handle it competently.
The discomfort you described and the investigations you reported make me suspect that you are going through terrible stress
after you adventure with bf. This stress can cause all the above symptoms you described.
As I mentioned in the beginning your sexual intimacy by itself will not cause any of this, but worrying, feeling guilty and tense could be producing all the symptoms.
Let us know how you feel. Feel free to ask if you have additional questions or doubts.
Dr. Ashok Koparday