Originally posted by Precious I have been using this lens for about 6-7 years,
If OP has been using this lens for so long, mostly at f2.8, as stated, then OP probably already knows about how shallow DoF is in macro range, especially at apertures wider than f5. And OP probably knows that no lens is optimally sharp wide open. So, sure, it is possible that OP is having problems with shallow DoF (which could be further connected to misfocusing), or that OP has expectations too high, but after 6 years of use? They probably have a good idea of what that lens delivers.
I see OP already checked for smudges on the front or back element.
Make sure there is no filter mounted. Check the mount and barrel for any signs of damage. The 100mm macro is an amazing lens, it is odd that it would develop a problem. But lenses can get damaged over time, especially if they are used very intensely.
Another thing to look at is PP - have you maybe changed jpeg options or lens profiles? Started using another PP software?
You might want to take it to a specialized repair shop and have them look at it. They usually charge a certain flat rate to just look at your lens and give you a quote on what actual repairs would cost. Hope you get it sorted out.