No, you can't assume they will call you. Though they usually will. If this much time has gone by, you need to call the office and request the results of your test.

You paid for it, it's not an adversarial thing, its that sometimes thing slip through the cracks. When you call for the results, make sure they understand what has happened since you were in to see the doctor and ask what you should do.

There is an article written by Merely Me, a writer for this site, called, Bleeding In Between Periods: What doe it mean? It may help you understand some of the conditions which can cause such spotting. It's impossible to know why this is happening without a doctor's examination, and I hope they can help you find the cause of this.