Weed causing stomach pain?

I’ve been smoking weed for about 4 years and I’m a heavy user. I’ve always had a weak stomach and haven’t been able to take birth control or even vitamins but for a year or two now a stomach pain and nausea wakes me up early in the morning everyday at around 4 or 5 am and my doctor told me that weed can change the way it affects chronic users. Do you think it could be the quality of the weed I’m smoking or just weed all together or could it be something else and if it is the weed does any have any tips on quitting