I am in my early 40s and have had several children (youngest now 8). I am 'reasonably slim' - have a BMI that is on the low side of normal for my height and exercise regularly. Would like to lose a few pounds but nothing dramatic. I've been trying to improve my core fitness so I've been doing a 10 minute online core workout everyday for the last 4 weeks. It's a course so the exercises vary- not repeating the same ones everyday. When I stand, my belly is pretty flat, just a small bump that I would like to get rid of.
However when I plank or put stress on my core it makes a really weird shape. Like it all gathers at the front in a big mass of muscle (and admittedly a bit of flab
but mainly muscle) the pics don't show it quite as dramatically as it looks in real life. I've never seen this on anyone else and certainly not on any of the instructors I'm watching online 
I'm wondering if there is something wrong here - my midwife did mention separation of the stomach muscles after the birth of my first child. I'm wondering if this weird muscle mass is related to damage done during pregnancy or if it's quite common for a woman of my age and I should stop comparing myself to the instructors! I'm also wondering if there are certain exercises I need to avoid / make sure I do to avoid building muscle that protrudes outwards. I don't want to make it worse!
Thanks!