Def talk to your dr. Especially since you are wanting to do get back into an exercise routine etc. Remember you did only give birth 3 weeks ago. Your body is still adjusting. It could be that you are over doing it to soon and this is your body telling you that you need to slow down. Your dr will be able to help you with a plan that won't overload your body. Have you been eating properly also?
I would definitely get it checked out one with your GP first to have you iron levels tested and if everything is ok on that front then I would see an Optometrist and have them have a good look behind your eye and have your eyes tested too.
I noticed from another thread you have joined a gym..I would ask them to put your membership on hold while you have this checked out and advise them that you only had a baby 3wks ago and need to get the all clear from your Dr before you can start it up again...just to be on the safe side. I know with my gym I can suspend my membership without any penalties due to illness or going on holidays so I don't end paying for something I am not using.
Look after yourself 1stTimemumma..take it easy and be gentle on your body while it is healing