Hope this helps....
It kinda depends on how sesitive the test was- was it blood or urine?
Usually, with a urine test, youd test positive around about the time your period would be due- about 2 weeks after you conceive, if you ovulated mid-cycle.
I'd say you propably got pregnant about 2 weeks before the positive result- around 24th sept.
These dates also fit with your beeding- finished around 10th or 11th sept is that right?
If you concieved around sept 8th, your pregnacy test on 1st october (more than 3 weeks later) would probably have shown positive.
It's more likely the baby is your current husband's, but, of course the only way to determine 100% would be a paternity test.