From a person who has had back problems related to a weak abdominal core I noticed the same thing when I started. Do 3 sets of planks on toes and elbows trying to get to 60 seconds and side planks 3 times trying to get 60 seconds. Youtube it or find a physical therapist to show you how they should be done. Add in 30 ab crunches a day.

This can all be done in between beers watching tv at night in the floor.