I just responded to your post.....it's on page 2, so I wanted to make sure you saw it.
I got severe OHSS before my ER - it is awful!!! So, yes, you can definitely get it just after trigger - it is actually worse when it's after trigger and before your ER.
Take care of yourself and call your RE if symptoms get worse....6lb weight gain is not normal....it sounds like you could have a bad case brewing.