OP - no, feeling like that is not a normal part of life, go to your GP. Get help now, for Ds's sake, he gets one childhood, a depressed mum who doesn't see the point of playing with him is no fun, of course he doesn't want to play with you now.
Even if you had got pregnant last year, you know it'd be at least another 2 years before they played together, you'd still be having to be the one to entertain your DS on holidays etc.
And then think about strategies for having an only, lots of holidays with kids clubs or centre parcs type places so your DS can make friends - holiday with other families with similar aged DCs so they can play together. Make weekend plans involving setting up playdates for his school friends, you will need to be more active in planning things, but that doesn't mean family life is over because you just have one child.
What it does mean, is you can dedicate your at home time to him. your holidays can be all about him, your weekends, all about him. You have the time and energy (and much more money) to give him the best possible life - but only if you are in a fit mental state to do it. Feeling miserable is not normal. Get yourself in the best shape you can. If you couldn't walk very far due to knee pain and there was medicine and phyiso available on the NHS to fix it, would you say "oh, I can cope driving everywhere, DS doesn't need a mum who can run around with him, I don't want to waste NHS resources getting this sorted." or would you just go book at GP appointment? Your depression is limiting yours and your DS's lives, thats just as worthy of treatment as a physical problem.