What Causes Light Periods?

Okay so I had sex almost seven days ago & me and my partner didnt use protection but he got out before he came I don t know if he might have came in a little or not but I ve gotten my period yesterday and it was very light unusual then the rest then as of now when I went to the bathroom it was no blood on my pad but a small spot of it and in the toilet. Could it just be a cycle change