project not straight

[Nicolapen]

I am crocheting the single crochet spike stitch. I have done a few rows and the edges are slanting, got the same number of stitches each row. Also the rectangle is slightly arched and not completely flat and I struggle to keep it flat when crocheting stitches. Could this be an issue with tension? I am using the hook size recommended for the yarn. 

I struggle with getting a consistent tension 

[Granny Square]

It looks like you are gaining stitches at the end photographed- is the count of your last row what it is supposed to be? This is a really common problem, and also sort of harder to tell with variegated yarn.

It also looks like you are stitching super tight as you mentioned, but that would have to do with the curl, not that the edge is slanted.

When I was a new crocheter I crocheted very tightly and had to teach myself to loosen up.  Do you use a tapered or in-line hook (google tapered and inline hook if you don't know what the difference is).  I used a tapered hook, and solved the tightness issue by learning to form my stitches further up the hook, beyond the taper.

[Nicolapen]

I thought I had the same number of stitches on the last row as the first but you are making me doubt myself. I have now frogged the project and started something else so will never know. Decided the spike stitch was too complicated in terms of getting the right tension.