Lifting
Recently, I took my dog to the vet and lifted her up on the exam table without thinking first. She weighs 50 lbs. Oh, the pain!!!! I felt something pull in my stomach and had major cramping for days. I went to the ER when I couldn't bear the cramps anymore. They took x-rays and said that the cramps are actually muscle spasms and I strained my abdominal wall. My doctor never told me how much weight I'm allowed to lift at 4 months out. Did your doctor tell you how much you can lift?
I am back to my full exercise program (resistance training/bodybuilding 3-4X per week and cardio 6X a week). At 4 months you should be able to lift any amt. The pain sounds just like a pulled muscle. Rest up and then consider starting a regular training program to help make you stronger!
Chances are you just lifted the dog up incorrectly. That could happen to anyone (regardless of strength or weight loss surgery).
Hope you feel better soon!!