Implantation Bleeding or am I going to start?

I’m not sure how many days late I am. I’m on day 41 of my cycle. I believe I ovulated late somewhere between sept 9-15. I’ve been having cramping in my lower abdomen/uterus area for about a week accompanied with lower back pain. Yesterday My uterus felt so heavy like it was full of fluid! Today I’ve been having Lowe back cramps (like I’m going to start my period) and had soMe spotting this morning. I have a blood pregnancy test today at 5pm but I’m wondering if I should cancel it.