I am a SAHM to a 13 month old girl and I can relate to some of the things you are feeling. I left a high paying, high intensity career to stay at home with my daughter, so my life has changed dramatically in the last year. It has been very difficult at times for me to figure out just what my purpose is in life now since I have no project plan or goals or meetings to keep me "on track."
I have to agree with some of the other people that said getting out of the house at least a few times per week is key. I was always going somewhere when my daughter was very young (the mall, Target, a walk around the block, and especially the gym). Now that she is a little older and walking, it is sometimes harder to go shopping with her since she wants to be down running around. When the weather is nice (which hasn't been often lately unfortunately), we have started going to the park. She really likes to be outside. I would definitely recommend joining a gym, health club or the YMCA so that you can get some exercise and your little one can play with some other kids giving you a little bit of a break. I think that might help you to feel a little better.
One other thing that someone else mentioned is getting a babysitter or putting your child in daycare for a few hours once or twice per week. I TOTALLY recommend that too if you have the means to do it. When my daughter was 8 months old, I started putting her in a Mother's Day Out program at my church one day per week from 9-2:30. This gave me an opportunity to do some things for myself. I also spent an hour each week of that time volunteering so that I was giving some time back to others in my community as well. This helped with the idea of having a purpose other than feeding, diapering and playing with my little one (all of which are highly important as well, but sometimes just don't feel like it). I also attend a ladies' bible study at my church one morning per week for 2 hours. If you're a Christian or would just like to spend some time learning something new, this would be a great way to get involved with other women and make some new friends. I'm not sure where you live, but I would be happy to give you the info on our bible study.
I haven't had any problems with actual depression, so I can't speak to what that would be like. But, I will say that it isn't something you should be embarrassed about. There seems to be a stigma associated with it that is entirely inappropriate. So, if you need medication to stablilize yourself until you can get back on your feet, then by all means do it! You have a job to do in taking care of your baby and you can't do that if you aren't healthy. Explain your symptoms to your doctor (even if it's a new one) and let them determine the next steps. If you think you can try some of the things suggested in this forum and do without the medication until you've given some other things a chance, then great. But, if your doctor suggests trying it, then you should. Take care of yourself so that you can take care of your family! Also, if you can manage it, take some time to yourself. Go get your nails done one Saturday and let Dad watch the baby. You'll feel like a new woman!
Best of luck to you. I know this parenting thing is hard, but it's so worth it. Hang in there...you're doing a great job!