Singer 99k stitches gone weird

[Emerion]

My Singer 99k was stitching beautifully until I sat down to carry on with my project yesterday. Nothing happened to the machine in between yesterday and when it was working. The stitches are no longer straight, each one is placed at an angle to the line of stitches. The tension looks wrong, like the needle tension needs to be tighter or the bobbin tension looser. But I haven’t touched the tension by accident  ( I have a note of what it should be). I have since changed the needle, changed the thread in both the top and bobbin reels, cleaned between the tension plates with knotted string, removed and reset the presser foot. If I turn the needle tension up nearly all the way to the top, the tension looks a little bit better, but the stitches are still askew. And it really shouldn’t need to be that tight. I’m stuck. Any ideas?

[dolcevita]

Presser foot pressure? If the fabric isn't held down firmly enough it could be that the fabric is twisting a little as the feed dogs take it through.
My 99k does produce a straight stitch where the stitches are at a slight angle, almost a bit like an embroidery stem stitch, if that helps.

[BrendaP]

It's normal for 'staight' stitches to be slightly slanting, like in this image

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It's because of the way lock stitches are formed.  The bobbin thread wraps around the needle thread each time.  If the tension is right those wraps will be hidden between the two layers of fabric but it often shows up as slightly slanting each time.  FIrmer fabrics and thicker thread are more likely to be noticed

[Emerion]

Thank you for the suggestion. It doesn’t seem to have made any difference - maybe a tiny bit. The stitches were arrow straight before today, it’s one of the reasons why I love this machine, they are normally so perfect. I’m not a subscriber so I can’t show you a picture.

[dolcevita]

The stitch from my 99k looks just like Brenda's. It's perfectly functional and strong, just a different look.

[Emerion]

Hello again, I’ve gone quiet due to terrible teeth issues. Had to wait for a dentist due to COVID, now but fixed and human again.
So, I  can see that those slanting stitches are fine @dolcevita & @BrendaP, not meaning to suggest they aren’t. But mine really were absolutely straight until this problem started. I’ve looked at several old projects of mine, and they are all the same straight stitch. So, what could cause this drastic change to the setting of stitches? I’m worried that something has “shifted” or broken in the mechanism. I’m thinking it will have to go into my local Singer shop when lockdown ends.