My Situation........ In Between Relationship. LMP 2/1/16, Cycle Runs For

my situation........ in between relationship. LMP 2/1/16, cycle runs for 3-4 days. Unable to determine ovulation date i can be irregular my cycle days range from 28-31. Guy#1 intercourse 2/9. BF back in town and we decided to work things out intercourse 2/10 and 2/14. on 2/17 had papsmear, pregnancy test was negative. Guy #1 comes back 2/18 trying to apologize and try again/ few drinks etc led to the unplanned, totally cut him off and ended everything after he decided to reveal he was married.2/20 BF back in town and moves back in, we had intercourse. I went to DR on 2/29 and had IUD placed, pregnancy test negative. on 4/20 i go for 6 wk IUD check up and positive pregnancy test, sono results put me at about 9-10 weeks. I'm so confused and seems like the doctors are also. I was given 11/12 as due date but that seems longer than expected. the pregnancy calculator gives me conception date 2/15 and due date 11/7. Can anyone help me gain any insight here?
Would one of these prenatal tests really be worth it?