TextEdit color change

My textEdit file reopens on startup with gray text on black background on one line. It was black text on white background before I shut down last time. I have my macbook (10.9.1) set to reopen windows from last session so window reopening is no surprise but text color changes is quite strange. It happened twice this past week. I live alone and no one have access to my computer besides me so this is really strange. I had printer & internet sharing on for a long time (I forgot when and why I had them on) but turned them off just now. Firewall is on also. Could it be someone had access to my computer from other location or it was just a glitch?

I'd really appreciate anyone who can help resolve this mystery. Thank you so much.