5 months without period and not pregnant.

I am 18-years-old. I'm 5'7" and weight 175 pounds, but people say I look like I weight 130. (Not so sure if I'm overweight or not 'cause they say I don't look fat for being 40 pounds overweight than what the chart says I'm suppose to weight) And I never have trouble breathing or doing exercise. I do have cholesterol, but I got it from my dad's side of the family. Approximately 5 months ago, I met this guy that I became really close to. We used to have often intercourse and it was often really rough, the thing is that whenever I have sex it pushes my cycle back to a week each time I do it and I only had sex between 3-5 times, so I expected my period to be at least a month late. I was wrong. It's been 5 months since then. I've taken 4 pregnancy tests, 2 from doctors, and they all came out negative. Sometimes my stomach really hurts a lot and I get nauseous a lot and stuff comes out of my vagina, as well sometimes. My stomach doesn't look any bigger than it did 5 months ago, but I am worried about my health.

Can anyone help me understand what's going on?

[Dr Santos]

Hi,
How are you? What other symptoms are present? It is good that the initial tests and evaluation by your doctor were all normal. Aside from pregnancy, absence of menstruation or amenorrhea may be caused by a variety of issues. This includes hormone changes, birth control methods, stress, poor nutrition, medication side effects, extreme weight loss, depression, illnesses, sudden weight gain or being very overweight, thyroid gland disorders, and tumors on the ovaries. If this persists, additional diagnostic tests such as imaging studies may need to be done. Take care and do keep us posted.