Another pregnancy is definitely a possibility, so a test is a good idea! :) How are your nipples feeling? If they're suddenly super tender when you nurse, that's another pretty good sign that you might be pregnant.
The erratic feedings are very likely to trigger the return of your cycles, and as another poster mentioned it is possible to get pregnant before your first post-pregnancy period if you happen to catch the egg from your first ovulation. It could also just be your body starting to shift back into gear because with the drop in feeding frequencies - some breastfeeding women menstruate without ovulating for their first cycle or two. This is more likely to happen if your periods come back before 6 months post partum.
If you are indeed pregnant with #3, there is ABSOLUTELY no need to wean your 6 month old, unless you decide it's the best path for you both. Find a copy of "Adventures in Tandem Nursing" by H. Flower and read it - it's a fantastic guide for women who need to decide if they are up for nursing during pregnancy and tandem nursing.
I am now 12 weeks pregnant with #2 and still nursing my 15.5 month old - aside from the painful nipples we are doing quite well. I'm hoping my supply continues to hold out (and that she won't mind the taste of the colostrum when it comes in) because neither of us are ready for her to wean quite yet. Obviously, with a baby under one year old, if your milk supply decreases significantly due to the pregnancy you'd need to supplement with formula. But the concerns that a number of doctors have about breastfeeding during pregnancy causing pre-term labor are based on out dated information, as Ms. Flower explains in great detail in her book.
If you are pregnant and do decide to continue to breastfeeding your son, be sure to find a provider who won't cause you to continually second guess your decision.
Good luck!
HP