Brown spotting 6 dpo

I was of the pill for 3 months due to bleeding twice per month. All 3 cycles on the pill were 28 days. This is my 3 cycle off the pill. Last 2 cycles were 26 days. I went to my obgyn today, she said my bbt chart shows I definitely ovulated, right before my appointment I went to the bathroom and noticed that between that time (11:30am) and the time I left my house this morning (8:30am) I had brown spotting. It has now stopped. My OBGYN said it’s normal. But I forgot to ask her what could cause it. I’ve never had brown spotting before. Any thoughts. Here’s my current bbt chart.