Yes, absolutely, you should call the doctor!
Do NOT feel guilty. You cannot help it; your biochemistry is off balance. You didn't do it and you couldn't have prevented it. You didn't choose it, so no need for guilt.
You are in a tough situation, being alone, but you care enough about yourself and your son to reach out when you feel something is wrong--that makes you a great mom!
I suffered from PPD for similar reasons (biochemistry and isolation among others) and did not succeed in getting help for various reasons. I don't remember the first year of my daughter's life. It's mostly a blur of misery. I wish I could go back now and have the confidence to make sure I got help, but I can't. I will not get that year back--it's worth getting help!
Tell your husband--show him this post if it helps. He can be a huge support to you in researching, backing you up, listening to you and taking you seriously, even giving you a chance to take a five minute walk when he gets home.
There are safe antidepressants for breastfeeding; diet and accupuncture can help, too. Please ask your doctor to have your thyroid checked (TSH, T4, T3 and using the new standards for "normal" TSH)--there are very strong links between PPD and thyroid disorders, often triggered by pregnancy.
If you're overseas with the military or a company, are there any other moms who can just drop by for a chat? Believe me, I would do it in a heartbeat for someone else--we've all been there!
You can do this--you just need the right tools. Hang in there!