Without worrying you, i Too had the same thing happen to me when DS#1 was 4 months old. It started as sore joints and by the time he was 6 monthsd old i couldn't lift him, bend down, or undo jars/open drinks. After seeing a rheumatologist i was diagnosed with arthritis and put on voltaran and then stronger meds as that did not help to the point of being hospitalized for steroid treatment. Apparently, horomes that are pg related can cause this to happen and some people only get it mildly and then it goes, others like myself are stuck with it. Subsequent pg can make it go away for good, however mine got to the point where my knuckles are now fused and i do not have full mobility of them. I can't make a fist or completely flatten my hands out. Thanfully pg with DS#2 went well with no meds for pain, only inflammation. I have no symptoms (touch wood) as yet and in all actual fact can squat down which is something i haven't been able to do since before DS#1. So pg #2 must have done some good.
All i can suggest is speaking your GP and start with some anti imflamatories and hope that does the job.
Take care