Dog becoming aggressive/protective

I’m all shook up. I’m not sure what to do or think. I live in an apartment and about ten feet from our front porch is a pool pump room (the pool is about six more feet away). I leashed my collie mix and opened the door to go walk her. I barely had the door opened about 3-6 inches when my terrier mix notices a maintenance man at the pump room and bolts out the door. He pinned the man up against the building and the man is obviously scared. I start to come up and say “don’t worry he’s not mean” right as my terrier mix starts growling and lunging at his legs. The man instinctively put his leg up to kick him but I never saw him kick my dog. I immediately grabbed him by his harness, and he kind of calmed down and gave into me as I dragged him into the apartment. I apologized a thousand times and came inside and cried my eyes out. My dog Diesel has never showed this kind of aggression and has only shown slight aggression since I’ve been pregnant (I am now 24 weeks). I’m horrified that he is going to tell the apartment complex and have him removed. I know it’s probably obvious at this point but Diesel is a mix of mastiff and staffordshire terrier, he’s 55 pounds and usually sweeter than sugar. After that I locked him in my bedroom then proceeded to walk my collie and came back. Then I walked him and he seemed calm and normal like always.

My big question is, is this normal now that I’m getting further along? Should I be worried about when my son gets here? Or if he’ll ever turn on me? I feel like he was just being protective of me but it’s never been this severe. He’s usually just weird about people or dogs running too close to me, and that never happened until I was pregnant. He hasn’t shown any signs of aggression to my collie or my boyfriend or anyone I let into my apartment since I’ve been pregnant. I sat down and sobbed for a good ten minutes, I was so scared they were going to have me get rid of him. I love him so much and I can’t imagine life without him.