A shift from 29-30 day cycles to 25-26 days over a year is a real change worth checking. Recurrent painful boils near your vagina also need an in-person look, since they can't be assessed from a description alone.
Next Steps
Please see a gynaecologist for both concerns together, and mention your ankylosing spondylitis and its treatment when you go.
Health Tips
If a boil grows quickly, spikes a fever, or your bleeding becomes unusually heavy, please get it checked sooner.