cheezcurl what tone of voice did she use? Was it concerned/gentle/harsh/incredulous etc? (Sorry if you've mentioned this already).
The tone of her voice would be the key thing for me here. If she said it gently, sympathetically and not in a dictatorial way, making it sound like a suggestion rather than an order, I wouldn't complain. If, however, she made it sound as though she was telling you what to do, then I would be very annoyed (I didn't breastfeed either of my DCs by the way, but I believe 100% that you should BF for as long as you want to if it suits you and your child).
Ultimately, if you need the antihistamines more than your DD needs breastmilk, then perhaps YAB a little bit U, but I can see why you are if the lady was insensitive in telling you so!
Not sure I'd complain though. Purely because, having made a complaint to an NHS Trust about something recently (for "something" read "my 66-year-old mother dying while in their care from something that was totally preventable"), I can tell you first-hand that it will wind you up and make you very stressed, which is not what you need when you're breastfeeding and suffering from asthma/hayfever! However, that's another thread, and I don't want to hijack yours