What Could Cause A Positive Pregnancy Test Followed By A Negative Pregnancy Test?

Good evening. I got my period last on 31th march. From 3rd of April, i have been having unprotected sex. Later on 15th of April, i started having vaginal cramps, headache, waist pain, stomach pain,fatigue,twinges in the breasts and also heavy breast.i took blood test on 19th to confirm pregnancy, it came out positive. But on 20th, i did another blood test and it was negative.what should I do?