Why am I having stomach pain?

For the past couple weeks I’ve been having what i thought was cramps and headaches from not having a period. I then got a stuffy nose and started to sneeze. It only lasted 3 days. Now I’m having an upset stomach, pressure in my stomach and headaches off and on all day. Nothing too bad or to stop everyday things, just enough to frustate me. Im not pregnant because I took 2 tests when I missed my period and bith were negative. I think I may be coming down with something but I just want to make sure its nothing too serious. Can anyone help?

Info: 20yrs old, female, 200lbs