I'm 16, for the past around 3/4 weeks I've been getting severe Nausea throughout the day about every second day (I haven't been vomiting) it's like a sharp crampy pain and the pain is almost unbearable. My periods is also late by about 21 days, I am sexually active but I have taken pregnancy tests and they all came back negative, it feels as though I'm about to vomit but I don't vomit and it's a very severe pain, I have not been constipated or had diarrhoea or anything like that, does anyone have any ideas of what this could be