Almost 28 weeks and my scar is hurting bad...

So I have had 6 kids, this pregnancy will be my 7th and my last child I had to have an emergency c-section due to the chord wrapped around him 4 times! He will be a year old on March 3rd. I am 28 weeks now and about two days ago my c-section scar has been hurting and turning red on one part of the scar. And by hurt I mean HURT! I am very small too. I am 5’4 and only weight 122 lbs as of today. My belly isn’t big at all. So my question is why is it hurting so bad and is it normal? Should I be worried or call my OB? I am planning on a vbac and really don’t want anything messing that up! I hated the recovery from my c-section.