Progesterone after IUI spotting?

Hi!! I am hoping someone can help me with this some. I don’t know a ton about it. I did 3 IUI’s last summer and used progesterone after and never had any spotting or cramping. I am 12DPO now and 14DPT. This evening I started having some cramping and very light spotting when I went to the bathroom and wiped. Can you start your period even while still on the progesterone? Or have break through spotting? I’m just confused. I took a pregnancy test today and it was negative. Thank you for any and all opinions & help!