Blood Draw Help

Has anyone ever had a vein swell with a blood draw after starting the third vial? I usually use a butterfly and a syringe. The company I started with only uses butterfly and vacutainers. I don't have a bunch of experience in this though I don't feel incompentent in the area. I did a lab this morning and during the third and final vial filling the vein started to swell and distend at the insertion site. I released the tourniquet and immediately it started to dissepate. I removed the last vacutainer and the needle and observed before placing the cotton ball and pressure that the swelling had totally dissapeared!! I have been a nurse for almost a year now (I know, not too long ) and have never had this happen before and it kind of freaked me out. The patient denied any pain or discomfort. Any help or advice would be greatly appreciated so this does not occur again. Did I apply the tourniquet too tight? Did I miss a step that I just can't figure out? Thanks in advance..........