Weird responses that I give when my brain doesn't work

So you know how when you walk into a store, someone will come up to you and either greet you or ask you if you need help with anything. Well, sometimes I don't response correctly to their greeting or question.

Just yesterday, I was at the mall, H&M specifically. I was just looking through the clothes, minding my own business when one of the employees came up to me and asked me how I was doing? But because my brain wasn't working and I couldn't comprehend what she just said, I thought she asked if I needed any help finding something. So I responded to her "how are you?" with a "no." In my mind I was saying no to needing help, but in the outside world I was saying no to how I was doing. It was kind of embarrassing. And she stared at me for a second, unsure how to respond to my "no", and just walked away. I tried to avoid her for the rest of the time I was in the store. I always do that. I'll say no when someone asks me how I'm doing. Most times when you walk into a store, the worker usually asks if you need help with anything. I never want help finding anything, so my response is always no. So whenever an employee asks me a question, I automatically think they're asking if I need help, and I'll say no without even thinking about it, without even thinking through what they just said.

It makes it seem kinda rude of me and I don't want them to think that I'm a rude customer. But it's just... my brain doesn't work in social situations. It's not even that social of a situation when a worker ask me how I'm doing or if I need help, but answering a question is already too much for me.

Another weird thing I do/say is when someone says thank you to me for holding a door open for them (for example), I'll say thank you back to them. They'll be like "thank you" while I'm holding the door open, and then I'll be like "thank you" and immediately after the words leave my mouth, I mentally slap myself in the forehead. Thank you? Really? What's wrong with you? Now they think I'm mental or something.

I guess sometimes my mouth is way ahead of my brain and I end up saying stupid things. It's not that big of a deal; I'll probably never see that person again, and they'll probably forget my stupid response in a minute, but my dignity is hurt for a few minutes.