When the words after yes and no "echo" the words of the question, use a comma.
...Q Did you go with her?
...A Yes, I did.
...Q Were you the only one there?
...A No, I was not the only one there.
Everything else after yes and no takes a period.
...Q Did you go with her?
...A Yes. Someone had to help her out.
...Q Were you the only one there?
...A No. My brother was with me.
But and because make no difference.
...Q Were you able to be of assistance?
...A No. Because I got there too late.
...Q Did she come over later that day?
...A No. But she did call.
Happy punctuating!
Margie