2 years ago I started having this really bad lower back pain, and when I touched the source of the pain, there was a weird feeling knot. I went to physical therapy for it, and it hurt so bad I cried every day from it - and it was very difficult bending over, getting up at a normal speed, moving objects, etc. But a couple days after therapy, it just went away, and now It's come back. I'm in pain but I don't know what it could be. Should I wait for it to go away like last time?
I've also been using a heating pad on the area.

Thanks!

Typically pain is a warning sign that something is wrong. Best to get it checked out by a doctor or chiropractor.

It sounds like a muscle spasm.
They turn into a knot basically and often they can be massaged out.

I have chronic muscle spasms in my back and you can actually see them when they are at their worst.

Here is a link that describes them in brief and possible treatments.

I am not a medical professional,I am going by my previous experience.Your doctor is the best one to access the situation.

Muscle Spasm Symptoms Causes and Treatments (http://www.healiohealth.com/muscle-spasm-treatment.html)

It is likely that you are suffering a recurrence of whatever it was, the last time.
Early Sciatica presents in this way, sometimes.
The pain can be excruciating and long-lasting!

If application of an analgesic ointment or gel does not give you relief, you would need to go to a doctor: if not for treatment, at least to find out the cause!