Sad to say OP but YANBU. I bet it is because you are black.
If its any consolation, I think it tends to happen to anyone who doesn't fit in with the norm, whatever that is. I'm (very) white with long blond hair, and to a lot of security guards, customs officials, etc. that seems to act like a beacon going off. Its so annoying. I've had particular interest from non-white security guards for some reason. Just last week, I was stopped going out of a Nike store and my bag checked (by a black security guard). I hadn't noticed anyone else being checked, including lots of Asian women with pushchairs and children who were picking up clothes and discarding them all over the place. Me - smartly dressed - immediately pulled over. Boots and Superdrug are a nightmare for me, I will always have a security guard on my tail. I actually think I could find not-so-gainful employment as a diversion for people that do actually want to steal/smuggle stuff, as the security types are so damned intent on following me, they must miss so many other people actually with stuff.
Last two times I've caught a cross channel ferry, I've been pulled over especially on return to the UK to have my boot not only checked but full on pulling stuff out for them to check for hidden compartments. I'm always the one that gets extra scrutiny in airport security. I do suspect that I get pulled over so much so that other passengers can see that really blonde, pale people also get pulled over.
But the best one was the Asian pool attendant whose eyes immediately swivelled round to focus on me as I walked from the showers to the pool. He got off his tower-thingey, to come over and tell me specifically that I had to shower before getting in the pool. I'm like the palest person ever, and I had to point out to him that you simply couldn't see the water droplets from the shower on my skin because I was so pale. It was so embarrassing.
But I bet it is because you're black, and you're also young and female and on your own. Maybe you're quite good looking or dress slightly unusually or well. Anything like that sets the buggers off. You have my sympathies. I'm often told I don't look British, so perhaps its that. Not typically British enough looking, easy to follow one single female with no-one to stand up for her.