Something I have always wondered is, is it okay to use spit as lube?

The vagina and the anus can absorbe fluids pretty quickly and saliva doesn't have the lubricity to remain in the area for long which is why you begin to dry out quickly. A good quality lubricant will ease friction and last longer than saliva will. Saliva could cause infections is your partner has oral yeast infection (Candidiasis or moniliasis) but usually only:
Newborn babies
Denture users
Adults with diabetes or other metabolic disturbance
People with a dry mouth relating to side-effects of their medication (eg anti-psychotics) or medical conditions (eg Sjogren’s Syndrome)
People undergoing antibiotic or chemotherapy treatment
People prescribed oral steroid medication or steroid metered dose inhalers.
Drug users
People with poor nutrition
People with an immune deficiency
generally get this type of oral infection so it'd be relatively rare. More than likely something is changing your body chemistry and if your boyfriend has had sex with you while you had or were developing a yeast infection he can be reinfecting you. More than likely you are not clearing your urethera after sex (oral or PIV) and a small amount of bacteria is being flushed back up into your bladder. The best way to prevent this is to pee right after the fun is over! Also drink plenty of water abd never hold it if you can help it.
It really is better to find a good lube that works with your body and doesn't irritate you or your partner(s). It'll last longer and be much easier on your sensitive tissues.