Here is a good link with lots of breeding info, it explains the signs to look for to see if your female is gravid
Reptiles Magazine November 1996 Article
The swelling you saw may just have been a pre-ov swell or an actual ovulation. (Or it could have been nothing, or it could have been digestion.) Once the female has the pre-ov swell, the male courts and eventually breeds with her, then she ovulates, then has a post-ov shed and approx 100-120 days later, babies.
Here is a picture of one of my boas gravid:
Not the best pic, all my good pics are on a disk at home. Anyways, her back half (in the rear of the photo behind her head) is swollen and leaning slightly, her side medallions are facing up. Gravid females will lay on their side (sometimes before ovulation as well), their back half twists up. They will also lay in a somewhat loose coil over the heat (called heat-conservation position).
If you don't want babies (as it can be difficult to find them all homes, trust me!) then I would get that male out of there as soon as possible, even if you have to set up a temporary tub for him (cheap rubbermaids $5 at big lots, etc). You may not get babies even if you leave him in, but if you don't want them, don't take the chance.
How is the male acting when in with the female? Do they just hang out together? On opposite sides? Does he keep his head near hers and 'ride' her around the cage? Are their tails curled together? His behaviour is the best way to tell if she may become gravid at this point.
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