Your outfit is lovely. I honestly would not take the comment as a negative remark but as a compliment. You mention she is a very supportive ward nurse and so you know she is in your corner. Sometimes people have the words come out completely wrong but mean well.
It sounds like you have your hospital creds in place so the question comes down to whether her remark made you consider whether a change in style would benefit you in the workplace?
If you wear a lab coat over then wear what makes you happy under?

Honestly? You look lovely, but the outfit doesn't project authority, which might be what that person was referring to. I think it's a combination of several factors: the sweet colors, the flouncy skirt, and the floral pattern. Don't get me wrong -- you look terrific, but I would probably save this combination for non-work days. When you're relatively young, sometimes you have to work harder to look like a serious professional. Fair? Probably not, but it is what it is.

I think you look fabulous. Absolutely fabulous.

I did read StaceyMcGill's comment though and I agree with her. As fabulous as you look, you may not project the exact image you desire. I also agree that when you are young, you have to work harder to project a serious professional image ... especially in certain fields such as law, engineering, banking, etc. There may be more flexibility in your field.

I woud think abt it from the patient's perspective. Is the outfit facilitating or preventing a therapeutic relationship?

Yes, patients do expect a white coat. It hides a lot of sartorial sins.

Well, my patients don't expect a white coat - in fact one of them asked if I had spilled something on myself :p

One of them sits at the end of the hall to yell "NICE OUTFIT" to me every day. I'm pretty sure he's blind, so it's all the more touching he takes the effort.