Your first thing, I would say, anytime when take new drugs and they change your behavior, there will be side effects. Though, side effects that deal with your health and different patterns, you should stop taking it, or if possible try to take it in smaller amounts, taking it in a bigger amount can make you feel sick and such. I know about this because I was on one and I had to first take it in smaller amounts and first I had to get the pills cut or split them myself. Though, I have stop taking them due to there being no money for me to take them for my depression and anxiety.
The second thing, this situation is hard to say what you should do. Though for me, if I was in your situation, wait another week and see what happens in reality. Since I don't know the guy or given me anything to go off of, who knows if he is away because of personal issues or just rather do something else then see you. It is early in the relationship to know what is going on. If he is someone who has friends and I mean friends that he is close to, he may just be finding spending time with them is more important then starting a relationship. Of course, there is the idea of work, as what he does for a living, I don't know, but if it could interfere with seeing you, That could be a possibility. Personally, I know how some guys think and I try to stay away from them.
Third thing, I don't want to get into anything as in when did you give birth and such, because that is your personal information. Though, I will say, a lot of women do complain about back pain after. Epidural and spinal anesthesia have no effect on post-partum lower back pain. So you can knock that out. You should probably look into Omega-3. Omega-3 fatty acids are natural anti-inflammatories that can help ease your discomfort. If your pain lasts for more than six months after giving birth, you should visit a chiropractor or other back specialist. For more long-lasting back pain, the cause is often situational, such as poor posture, that started during pregnancy and was never corrected.
As for your stress, this is when I'm going to say your depression and back pain can all be causes of this. Personally, trying to have a relationship that is starting out, can cause more stress as well. All this is stressful for you and the hardest point is being a single mother. Now, I may have no been in your situation, but I know of the stress because I was a child of a single mother and saw all the stress it can bring. I won't say it'll get easier or anything, but you need to find a way and even just putting your little one down for a nap and crawling into bed and just laying there, relaxing can help. If you need to do something active to relief stress, I suggest doing some simple exercises, like sit ups, push ups, simple things. Do at least small amounts before doing more then you can.
You have a lot of changes in your life right now and people in real life could be looking down on you for being young and being a single mother, which I will say is perfectly normal to most single mothers. My mother has told me this herself. She had a harder time with having relationships with guys because most men don't want to start a relationship with a woman who has a child, nor would the mother want to introduce her child to her bf too early. But that you don't need to worry about as much with doing that with the baby as she is young still.
I would try and keep in contact with family and friends that will accept everything you are doing and it takes a strong woman to do and an even stronger women to prove them wrong.
I hope some of my words mean something to you, and you will now know you have a friend in me and I am here to give advice or even just to rant to me. Maybe perhaps, I could look at my cell phone plan and see if it's fine to text across Canada, not cost too much and we could exchange numbers to text. ^.^