Numbness in my left chest/pectoral area, around armpit and breast. No other symptoms, no lumps on breast. Has been going on for 2 weeks. Usually happens at night. What could it be?

Left chest:numb. This kind of one sided "numbness" involving armpit and breast can arise from compression of the brachial plexus just under the upper arm. The compression may be related to your position during sleep, and may be more frequent if lymph nodes are swollen in the underarm area. See your healthcare provider for an exam and testing if this continues.
See a Neurologist.
The area you are describing corresponds to the Thoraic T 3, T4, T5 vertebra in the back.
I enclosed above dermatome link for you to see how the body nerve system runs.
A specialist in the nervous system is a neurologist. This is a Doctor that will test the skin location first for soft touch then pin prick and begin to work out the problem for a diagnosis.