@notamumsymum
First of all, I want to apologise for what you experienced. Even though it was not my fault, but I do feel your pain, and no human being should experience this.
I am a white woman and my husband is black and since we got married, I notice this type of thing all the time. I am not sure if its direct racism (like the person following you was aware of their racism or if they subconsciously treated you different).
Things like this happen every day in the UK and other countries where you experience different reactions or experiences in situations because your not white. Most white people do not notice that this even goes on. My husband has been followed around shops, once we were in New Look and a security guard followed him. Other times when I have been on my own I notice that when Asian people leave Tesco, if the door alarms start going off, its an automatic let me check your receipt, but if this happens to me or any other white person they would never bat an eyelid and let me go without even realising the door went off.
Other times my husband has been subjected to racism on the bus by bus drivers and the worst is that airports where you are scrutinised. I know there are situations where things happen, and its nothing to do with your race and I also realise that white people are subjected to these things as well at times, but I have never been watched in a supermarket, stopped at an airport or treated poorly on a bus.
It has only been since we got married. I noticed all these things. We will be in public and employees in restaurants or passers-by on the street will be looking at us with such rage and anger for merely being together as if two people from different races being a couple is a crime.
Again, I am sorry you have had to experience this. I would call them on out on it and ask to speak to the manager next time and say that you did not appreciate being treated differently based on your race. I bet there were white people in the store and they were most likely not following them.