Temperature blanket started off with 238 stitches, after 11 rows of double crochet I now have 248 stitches.

First off, english is not my first language. I thought my english was alright, but I'm not so sure anymore with all the crochet terminology. I hope you can still understand what my problem is.

I wanted to make a temperature blanket for 2025. I just started, without counting the chain stitches I was crocheting. (I know, not the best idea, but i still did it.)

After some trial and error, i was done with the first few rows. I started to notice that my "blanket" was beginning to take the shape of a trapezoid instead of a rectangle.

I'm doing the blanket in double crochet by the way and chaining three in the beginning of a new row.

I think i might have worked into one more stitch than i was supposed to when coming to the end of a row. But I just don't get where i have to work the last stitch of a row. I have tried to watch tutorials but i still have no idea.

Can anyone help me?