Williams' shoulder popped out during a heavy tackle by Frans Steyn and he was forced to sit out the second half at the Millennium Stadium and see his side lose their grip on proceedings against the world champions.
"I just don't know yet what I've done. I've had an operation on both of them in the past to clean them out, but because I have never dislocated my shoulder before I'm not sure how bad it is," admitted Williams in his Wales on Sunday column.
"I went down, heard it pop and then when I rolled over on it, I heard another pop. So I'm assuming it has popped out and popped back in.
"But I'm not a doctor. I have a scan planned, so fingers crossed. I don't have a clue how long I may be out.
"It's as sore as hell at the moment, almost unbearably so. It's like toothache and I've had to take quite a few painkillers. I am hoping there are no major complications and that it is just bruising so I will be able to get back as quickly as possible."
Wales scored three tries against South Africa for the second time this year, but once again came up short after twice leading by 11 points. Williams fellow wing, teenage debutant George North, had a dream debut as he scored two tries, the first within five minutes of the kick-off.
But the Springboks gave acting captain Victor Matfield something to smile about as he celebrated taking over as the most capped South African player on his 103rd Test appearance with a magnificent third quarter fight back.
Tries from Matfield himself and replacement back row forward Willem Alberts hauled the visitors back into the game and the boot of Morne Steyn did the rest as the Springboks edged home in another close encounter after their narrow victory over the Irish in Dublin.