Odd. Have you checked your APN settings to see if you have a proxy set or something that might be interfering with Speed Test's results? Or, you could try visiting a speed test site in your browser and see if that makes any kind of a difference. Finally, go visit a T-Mobile store tomorrow and both test there and test on one of their phones. It may just be that T-Mobile is really congested or slow in your area.
If I recall correctly there are two $30 plans, one with 100 minutes voice and unlimited data (first 5 GB at full speed) and one with unlimited voice but very little data (10 MB, I think).
Are you sure you signed up for the correct plan? Sign in to My T-Mobile and check your plan and remaining data.
Your symptoms sound exactly like what my wife was seeing when she hit the 100 MB cap with the $50/month plan.