Its not likely that you'd get CFS after having sex.
However common bloodtests don't give any answer to having or not having CFS. If you have bloodtests that also show all your vit. & min. levels and possibly hormone levels it's easier to figure out, whether or not you have it.
Could you please message me the STD list that you have done.
The cause of chronic fatigue syndrome is unknown, although there are many theories — ranging from viral infections to psychological stress. It is not a contagious disease and cannot be transmitted by sex. There's no single test to confirm a diagnosis of chronic fatigue syndrome. You may have to undergo a variety of medical tests to rule out other health problems like depression, schizophrenia, eating disorders such as anorexia nervosa, bulimia, bipolar disorder, alcohol abuse or other substance abuse; current morbid obesity, and active medical diseases.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.