Good grief. I can't believe people (a) comment on a pregnancy when they don't know for sure, and (b) think it's OK to touch someone's belly. Aaaaargh.
First: you don't look pregnant.
Second: You don't have to throw out all your tunics.
Third: About that top in particular. This is where it gets iffy. I think it looks great, and let me reiterate it does not make you look pregnant -- but I confess that if this happened to me, I would be very unlikely to wear the top ever again. This is because of my personal feelings about pregnancy are not overwhelmingly positive. Pregnancy made me feel gross -- like my body was out of control -- and I didn't particularly like how I looked pregnant, either. (Some women look cute; I looked like a sweaty, shiny, bloated tick, albeit with thick lustrous hair). So if I wore a top that made people think I was pregnant, I would never be able to get that image out of my head.
Having said that, you are probably more sane than I am, so I think the better advice would be to just ignore the silly woman and keep the top.
P.S. No one ever touched my belly during either of my pregnancies, despite the fact that it was very clear that I was, in fact, pregnant. I've always been a bit sad about that, because I was so looking forward to giving someone a public dressing-down for daring to touch me.