When I received my ultrasound to confirm pregnancy October first , saying I had a due date of may 26 2013, and I was six weeks and so many days pregnant. I cannot recall when the date was a had intercourse with my boyfriend. He had a reversible vasectomy 8 years ago but never had it checked or maintained. I had intercourse with another partner, that for I knew did not lead to ejaculation, afew days or so after September 14th.
My second ultrasound at 20 weeks says I have a new due date of may 18th 2013.

My question is what is the true potential my second partner fathered my child..

It's not possible to calculate conception dates with any degree of certainty. Gestational dates are all Best Guess based on available evidence.
If there is uncertainty surrounding paternity then a DNA will need to be conducted.

My first pee pregnancy test was 6 or 7 days after the rape happened and the pregnancy test came back positive... is that even possible I thought it would take two weeks to produce a positive test.
Im deathly afraid to cause all this trouble and upset my boyfriend and home life for a chance

Normally it can take up to 6 weeks for a good urine test. 6 days if very unusual.

But you really need to do a DNA test to be sure, conception dates are good , give or take 1 to 2 weeks, so there is no exact day ever known.

I looked into dna testing and it is very costly... is there help for someone In my position. I was too scared to call the police ordo anything... I wanted to forget and move on

DNA testing is not that costly,
It can be done at private labs fairly cheaply.
And sorry what position? You had sex with two people, as for as anyone knows except you, it was consensual sex, since no report of rape was made
It is not too late to call police of course.