Did i need a pulpecomy? i had no symptoms at all: no pain, no discomfort. i was there to get fillings, but dentist suddenly told me i needed it.

Only your DDS knows: Unfortunately, without being able to see x-rays of that tooth and\or clinically treating that tooth ourselves, we have no way of knowing whether or not you needed it. However, why doubt your dentist's treatment? He\she should find it easy to show you the x-ray & explain to you why it was necessary. Simply ask and i'm sure you will be assured with the answer. It is often necessary even w\o pain.

Pulpectomy=rootcanal: Often, dentists discover the need a root canal treatment due to deep decay that enters the nerve space (often the nerve "died" a long time before therefore no pain) or they may "see" an abscess (infection) at the tooth root on an x-ray or the tooth may be significantly broken down that they need to perform root canal treatment to be able to restore the tooth.

Wrong assumption: You assume that without pain or swelling that you did not need treatment... That's not always the case. High blood pressure, diabetes, and many other illnesses don't always have outward symptoms... You wouldn't wait until you had a stroke to be treated, would you? Have faith in your dentist.

Probably Yes: A decay penetrated nerve is a problem. You may now have an abscess even without having pain. The infection of the abscess will not pain you as long as it is draining. Once it cannot drain, then it swells and hurts. Infection is nothing to play around with since infections always enter the blood stream and goes everywhere in the body the blood stream goes.

Pulp exposure: While performing the procedure, the pulp was probably exposed. The pulp may not have been diseased, but if it is exposed, it will become diseased it most cases. Hence the need for a root canal. Sometimes, if the exposure is pinpoint small, the dentist may pulp cap with a medicated material and observe. It all depends on the size of the exposure. Don't hesitate to ask your dentist about your case.