Possible Pregnancy scare? Or stress?

In February I had a period on the 9th through the 11 then had sex with the SO 14 and 15 and on The 19 had spotting then on the 20th a full blown period again that lasted til the 24th and was heavy. Now I've been having back pains and stomach pains and feeling nauseated maybe it's stress or the flu? Any suggestions on what is going on!