You need to call the office and speak to either the office manager or doctor, preferably both, and tell them exactly what was done. If this is a hospital-affiliated clinic, contact hospital patient relations (call the switchboard, tell them you want to lodge a complaint against an employee and have them direct you to the right person or department) and file a formal complaint.

If you are serious about persuing a lawsuit, then tell them so and see if they will offer anything. However, I suggest you not say something like this unless you are well prepared to do so and intend to follow through if the response they give you is not satisfactory. You don't want to open that can of worms unless you're well and truly serious.

In the mean time, assuming this woman truly is the culprit, she should truly know better than to do such a thing. I don't know if technically what she did was legally wrong (gossiping about seeing someone come through her doors, versus handing out personal information out of your medical file), but she nonetheless deserves a strong, official reprimand from the office/clinic/hospital. Regardless of the legality, it's just not ethical.

However, all this only applies if you're okay with her getting in serious trouble. Just because she's a single mother doesn't excuse her actions. You can always talk to her first, explain how upset you are and the consequences she could face if you raised this issue to her superiors and see how it goes.

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