I had sex on the 10th of December, got my period on the 14th - same month. Lasted 5 days, heavy flow. I was still worried so I got a home pregnancy test and took it on the 31st of the same month - so 21 days after sex - it was negative. Now, four days before my period is supposed to come, I feel nauseos, and my breasts hurt. Can I be pregnant?

Sounds more like PMS.

Well you did already got your period so I think your clear

Possible, but not likely. Not likely that you would have ovulated right before your period given your cycle length. You had a regular period, and you had a negative test result 21 days later. Wait and see what happens when you would expect your period.