Transcutaneous Electrical Nerve Stimulation (TENS) units may be helpful for a wide range of painful conditions, with good data indicating decreased pain intensity in patients with endometriosis, menstrual pain, dymenorrhea, vulvodynia, interstitial cystitis/bladder pain syndrome, low back pain and other musculoskeletal causes of pain. We think that TENS units work through a mechanism called gate theory. Essentially, TENS is a non-pharmacologic stimulus to block the ascending sensory input and prevent pain signals from getting through, which decreases the perception of pain by the brain. This can be used for acute pain episodes (just use when you're in pain) or for chronic pain (use daily or several times per week). There is some data that regular or daily use of TENS can change the function of the peripheral nerves in the spinal cord and in the brain to decrease pain perception long-term.
You can adjust the intensity of the TENS signal. You should try to use a high enough intensity that the sensation feels like a strong tingling. This sensation is what blocks other pain signals. However, you shouldn't feel painful, and you may need to adjust the intensity from day to day depending on how you are feeling and how your body responds.
You should NOT use a TENS unit if you have a pacemaker, implanted neurostimulator (interstim for overactive bladder), or in-dwelling pump (such as insulin pump) as the electrical current could interfere with the function of your device.
Your physical therapist can help you use TENS most effectively and give you individualized guidance. This website has some excellent information about general TENS use to get you started.
You don't need to buy an expensive or professional-grade TENS unit. There are a number of options are affordable and have a nice variety of TENS settings that can be good for most locations. Your physical therapist may be able to provide specific recommendations.
For pelvic pain, you can try placing the pads on the right and left lower abdominal wall (just above the pubic bone on the right and left lower back (on the sacrum just above the tailbone on either side). But there is no right or wrong place to put the pads, so try a few locations to see what works best for you. Your physical therapist can make individualized recommendations to best address your symptoms.