My girlfriend felt really sick and shaky after sex and she really felt bad so that she only went to bed and had an advil (ibuprofen) pill. why is that happening?

Depends on details: If these symptoms began after orgasm, she may have postorgasmic illness syndrome, which is thought to be due to an autoimmune reaction. If her illness begins during sex and includes a headache peaking at orgasm, she may have coital cephalgia, a migraine-like illness which is treatable with propranolol. If she's not having orgasm, it's worth finding out why. Discuss with her doctor.