I’m a very enthusiastic practitioner of Iyengar-yoga, but can only afford a limited number of classes.
Recently, when practicing Sirsasana (I always do it against the wall, trying to build up strength to leave the wall), I sometimes find myself having a pretty nasty stomach ache afterwards
-sometimes it lasts half an hour, sometimes all day/ night. It only hurts when I breathe, which makes me think it’s got something to do with my muscles…
I don’t eat 3 hours before and half an hour after my practise and only drink a little.
Has anyone experienced this?
I’d like some advice on what I might be doing wrong, since I adore Sirsasana and can’t wait to get off the wall!
Is the pain you are having located just below the left breastbone about three fingers left of your sternum?
Actually it’s mostly to the right and on both sides of the lower abdomen?
Wait four hours instead or three after having eaten.
I also upset my stomach if I only wait three.
I have heard it before, it can be due to the movement of gas in the lower abdomen, in Shirshasana the apana moves upwards towards the mooldhara which can give pain if there is some blockage in mooldhara chakra, try to take deep breaths in lower stomach while in head stand and dont do it for lkonger than 2 minutes in the begining,
Below the rib cage or no?
below the rib cage on the right side, yes 
Ascending colon? Cleansing from the inversion? Spastic colon not alluded to by you in your posting? Other digestive issues you’ve not outlined? Incorrect actions creating issues with abdominal muscles? Anyone’s guess.
Best broached with your teacher if it is ongoing.