Adding unwanted stitches in stockinette help needed

Hi, My name is Ellen. I have been knitting for almost a year now and have not had any problems working stockinette stitch until now. I am currently working on a new cardigan. I did a long tail cast on of 120 stitches and worked the first 5 inches in garter stitch, no problems. Changed my needle size to size 8 and began working in stockinette stitch and have knitted approximately 23 inches into it. Well I counted back my rows and noticed that I had 124 stitches instead of 120. I assume that I was doing yarn overs and ripped it back. After working on it, I had only 119 stitches. I continued working with the intention of increasing a stitch on the end. Well after working a number of rows and being diligent not to carry the yarn over the needle and keeping it in back, I’m back up to 122 stitches. I am at a loss of what I could I am doing wrong and am truly frustrated. Does anyone have any ideas as to what I could possibly doing wrong? Any help is greatly appreciated. Sorry so wordy. Ellen Victoria PS. now I have two extra loops on the end.

You may be adding extra sts at the beginning of the row. When you turn after a knit row, be sure to pull the working yarn down from the needle. If you pull the working yarn up from the needle, you’ll see what looks like 2sts but is actually only one. If you knit those sts, you’ll be adding in an extra stitch each time.
Try putting a marker (this can be a loop of contrasting yarn or a store bought marker) on the needle one or two sts from the end. Slip the marker at each row. Then you’ll easily be able to see if you’re adding sts at that end.

Thank you very much for the quick response, I think you are correct. I’m going to add the markers like you suggested. Appreciate your response. Ellen

Usually it’s on the garter stitch section that you’d add stitches at the beginning of the row. Put part of your stitches on another needle so you can spread them out and check to make sure you haven’t added a yo in the middle of the row, or maybe knit a stitch but didn’t take the old one off so both sts got slipped to the right needle. If you find something like that, you don’t have to rip out, or certainly not to the beginning, only a few rows. There’s some videos on how to fix mistakes on the Tips page - you might be able to just drop an extra one off and work the extra yarn into the next stitches, or drop down to fix one that got knit twice and just bring up one of the stitches.