Why does my stomach feel bloated and have cramps?

I’m underweight , last week
I took day off from job & ate all day, I eat small portions every hour. I felt fine then next day At job felt bloated by lunch and at night cud barely eat my stomach was sick too pepto was in bed and felt slight pain discomfort. Yesterday I took another day off from job , ate all day then today I get home 5:40pm, eat some soup, 15 min later eat bowl of beans with juice, 20 min later try to eat a small piece of pumpkin pie and stomach feels full but empty like air and left side was hurting like cramps and I have an app that says period in 8-9 days. I started drinking a weight gainer protien shake daily at night. But not today I drank a normal protien banana shake in morning. At job I eat very little due to no breaks only a lunch at noon. I work 7am to 5pm. I ate half a burger for lunch and eggs for breakfeast with bread with cream cheese. I have medicine I can take 4x a day one hour before I eat to help food digest better and forget to take it.
Why does my stomach feel bloated and have cramps?
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