Right, having shed a few tears I'm now calmer and can ask you all about this. I really don't know whether they were right or not, so I'm perfectly open to be told they were and if so I'll just get a grip and forget it.
Today we went shopping as DD needed new pijamas. While we were shopping at a store (don't know if I can name them) DS, who is 6 weeks old, started crying. I had fed him at home so that he wasn't hungry for a little while at least, so I took him out of his pram to check whether it was maybe wind. It wasn't wind, he was hungry so I looked for a quiet area in the store, sat down in a low level piece of furniture and started feeding him. I was very discreet (nothing could be seen), no one could see me, there were only two other ladies in that area. A female member of staff passed by me as I was next to one of those "staff only" doors. She saw me and smiled, didn't say anything.
A few minutes later a member of security approached me and told me, very politely, he had been told I was here and he had been asked to tell me I couldn't breastfeed in the store. DH was near now (he had been queuing to pay), heard him and asked whether he was sure that was right. He said yes, so DH politely asked him to speak to the supervisor.
By now I had already finished feeding DS so we just waited for the supervisor to come. When she arrived she told us a member of staff had told her I was there and that I couldn't breastfeed the baby in the store. To be honest she didn't choose the best approach (tone and manner), we asked whether she was sure that was right and she said yes, that this made customers feel uncomfortable 
She then changed what she had said a bit and now it hadn't been a member of staff who had told her but a customer who had complained about me breastfeeding. I could see DH's face by now and I was already feeling a lump in my throat so I asked DH to leave it there and to just go.
DH, however, asked her again how things had been then, whether it had been a member of staff or a customer, etc, to be honest I just wanted to get out. She had the lovely idea to speak quite loudly the whole time so I felt people were looking at us, agh! DH asked her for her name so she went away with him and DD to give him a card and meanwhile I started putting DS in his pram. The security guy, who stayed with me, then told me that maybe it had to do with the fact that no food or drinks were allowed in the store 
And then he said he had been breastfed himself and he had breastfed as well (I think he obviously meant his partner) so he had no problem with it but this was how things were. I could only feel the lump in my throat getting bigger and bigger so I just said "This is really disappointing, to be honest" and started to walk towards the lift.
Were they right? This never happened to me with DD (2.4 now) and in 6 weeks it's happened to me twice at two different stores. Would it have been any different had I not sat down and directly walk around feeding him? Would they have been able to say the same? I clearly remember feeding DD walking around places all the time so maybe things have changed? Should I complain with their head office? I really want to know if they were right so as to not do it again or plan whatever outing I have to do better. I felt horrible and cried from the moment I stepped out of the store, but I'm prepared to be told I'm just being hormonal and overreacting.
Thanks and sorry for the length!