What is my due date?! Please read!!

My last period was Dec 18th and that is also the day I started birth control.

Towards the end of December we were trying again to concieve. The first week of Jan I stopped taking my birth control.

Jan 20th I started spotting. It was pink and then brown.

Found out I was pregnant 4 days ago and I just can't figure out when I got pregnant. Much less my due date!

Mind you my fiance is in college and didn't come home until the weekend of Jan 29th. We started trying again.

Can any if you ladies please help me?! Its driving me crazy!

Thanks in advance!