FTM OVEREACTING?!

Hey mama's! My little girl is almost 8 months old and refuses to sit. If she's happy enough she will and she can sit unassisted, but still falls over and stiffen up her legs to stand. She can army crawl in circles but that's about it. If you stand her up, She can walk (assisted) and when she holds on to your hands She can pull herself up to stand. She babbles like no tomorrow and can roll like a champ. I'm just worried she's falling behind? maybe she is just working on the walking? first time mom here and no idea what to expect. I'm trying my hardest to help her learn and be a good mom.