Sudden allergy to cows milk????

[Lucyand2]

I posted yesterday about my Dd being ill. She had been sick when I went in to see her in the morning, she vomitted after her morning cows milk and she then had terrible diarrhoea for the rest of the day. Dd hasn't had a temp, she's not eaten anything different and seems absolutely fine. Today we carried on as normal until about lunchtime when I let her have a few sips of my milk (She'd just been on water since yesterday)
Within a couple of hours she was very sick again, no diarrhoea as yet but she is very farty. Dd has never had a problem with cows milk before but I'm suddenly wondering if she's allergic?
Is there a time period I should give her before offering milk (if it's a simple tummy upset) or is there some way of finding out if milk is causing it?
Dd is 16 months old and stopped breastfeeding nearly 2 months ago now.
Any help would be much appreciated as I'm really worried about her!