No sperm, no pregnancy. Full stop.
Periods are weird. Sometimes they're heavy. Sometimes they're light. Sometimes they're early. Sometimes they're late. Sometimes they're horribly painful. Sometimes they're not.
Seriously, universe, you could have picked a MUCH better way to tell human women if we are pregnant or not.
So - as your doctor said (I'm no doctor), it's probably stress. The school year is starting and life is changing and that can be stressful. Backache, etc. could be from a ton of things. Hell, you can get a backache from sleeping wrong. As for breast pain, you can get it with weight gain (such as water weight - are you eating a lot of salty foods and not drinking a lot of water? That can give you water weight gain).
You could be coming down with a cold, too, which could explain some of how you're feeling, or even the flu, which would explain the nausea. Nausea can also happen if you get panic attacks.
As for the homemade pregnancy test thing, it's utterly, 100% bogus. It's rather similar to the known reaction between baking soda and vinegar
The reaction happens because vinegar is acidic (pH of 2.4 or so). Your urine falls within the range of 5.5-7 pH or so.
Baking soda + any acid = that exact same reaction.
So all it means is that your urine is slightly acidic. Which falls within normal.
BTW, the baking soda + urine test is allegedly to predict the gender of a baby. See: http://forums.webmd.com/3/urology/forum/297
This is the problem with these kinds of bogus 'tests'. No one on the internet can get their stories straight.
Please stop cherry picking medical advice from over the internet - including my answer, I might add. Talk to your doctor if you are still concerned.