Scared it isn't his baby. Please help.

I had a fling that lasted a month in October. We had unprotected sex twice within the span of maybe 2-4 days. That's all. I had a period in December & January following the unprotected sex. I can't remember November & don't have anything marked on my calendar.

...I got back with my current partner in early-mid December. I had a period January 7th. Faint positive on February 7th--strong positive on February 9th. I'm currently 25 weeks pregnant & I haven't been able to shake this absolutely HORRIFIC fear that somehow I'm pregnant with another man's baby. Someone please tell me I'm crazy, or tell me what I can do to get rid of this fear?!

I'm due October 14th. We're both super happy, super excited. I don't know why I can't get over this anxiety, it feels horrible.