What could be the cause of stomach pains after eating and missed periods?

Since about January my periods have been irregular whereas always before that were consistent. In March I had my period twice, then 6 weeks later in May and not since then. During this time, I increased amount of exercise - running about 3 miles per day and light weight lifting. In addition, I am also a student so stress may be a factor. However, I have had severe gas, bloating, and constipation. It has gotten progressively worse overtime. Generally, when I eat something my stomach hurts. Some things do not bother it as much, such as popcorn but foods like Mexican and drinking beverages like beer make me really uncomfortable and in a lot of pain. The pain is slightly sharp and severe enough to interfere with my life. I’ve been to a gastrologist who is giving me meds, to learn what I have or what will make me better trial by error. I don’t want to wait months after trying different meds. Has anyone had similar symptons?