HIV transmission sharing a smoke

First I'd like to suggest that you get tested.  I recommend this because of your anxiety about sero-converting to HIV rather than because I think transmission in your case is likely.  Although theoretically possible to transmit HIV by sharing a smoke with someone who has bleeding gums or mouth, transmission needs an avenue to enter directly into the  bloodstream - cuts or abrasions in your mouth.  Hearing that HIV in your case is very unlikely will probably not be enough to fully relieve anxiety.  Talking to a physician and testing negative will hopefully be more effective in lessening your worry and concern.