17 Month old daughter, behavior problems or severe separation anxiety?

My daughter is 17 months old and has always had this separation anxiety from me. I'm a nursing, stay at home Mom and she is almost never away from me. On the rare occasion that she is being babysat or is with my husband when I'm out...she cries for a few min when I leave and then is fine again until she see's me...she runs to me crying even though she was happy right before I walk in. The bigger problem seems to be while I'm home with her. She ALWAYS wants to be held and if I don't she will literally cry until I pick her up. I've tried just letting her cry for a little, but she literally wont stop. She does great with naps and isn't overtired during the day. She has started to purposely bang her head on the floor when I don't pick her up right away. And sometimes, when we are sitting on the couch watching tv, she will cry and come to me and I'll pick her up and she continues to cry and trys to get down...but as soon as I put her down she flails around on the floor being upset. I've tried to even talk to her and ask her what is wrong and she needs to tell me...and she just keeps flipping out. I'm pregnant and due in a month or so, and I'm getting worried that I'll always be holding a newborn and a toddler...any ideas as to what is going on with her? Her 18 month check up will be coming up soon, and I will be mentioning this to her doctor...