Being your first log cabin, I think it is probably your seam width is off a tiny bit causing the previous 'logs' to be off and you are pulling the new log to match and causing a stretch. I say this because most of us have done the same thing in the beginning. If very minor, you may be able to press it flat. Also, be careful that you just aren't being over critical of yourself (another very common problem!).

Are you using batiks which are a tighter weave or is your stitch length too small?

When doing log cabins I use over size pieces, longer and wider, and trim them down. I am not the neatest quilter so I have to come up with ways that work for me. I do have a tendency to pull fabrics so I solved it by using the oversize and trim down method.

peace

Starching the fabric before you cut them into the strips for a log cabin helps keep their shape and doesn't ripple.

I found that in the beginning I was cutting my strips incorrectly, placing. My ruler over the bulk of fabric, as opposed to only the strip width of fabric

I was just sewing some log cabin blocks together and had this same situation happen. Some wavy strips as I sewed them.

I discovered that there was ever an ever so slight curve in the strip. The ruler I used to cut the strips was an old one. I picked it up not realizing it. The ruler was worn down and as I cut the fabric it created an unnoticeable curve until I started to sew them together.

This may also be a reason for wavy strips.

Sounds like you cut/sewed log on the crossgrain or width of fabric.
Cutting them on the straight grain (parallel to selvedge) omits the waviness when sewing pieces together.
Good luck!