On Sept 20, 2020, I received a flu vaccine in my left arm at my local pharmacy. The procedure occurred smoothly (no issues during the injection). The next day, I felt soreness in my left arm, which I've experienced with prior flu shots. Usually, that soreness would dissipate within day or two. However, this time, the soreness remained many days later. This had never happened before and I kept assuming the pain would naturally go away. When more than 10 days had passed, I knew something was not right.   On Oct 6, 2020, I went back to the same pharmacy where the shot was administered. I explained the situation to the nurse, who inspected my arm (she was not the same nurse who administered the shot). She did not see anything unusual and recommended ice and Lidocaine. She indicated that if the pain continued, I should see my doctor.  As the pain did not go away despite the home treatments and the passing of time, I visited my primary physician's office on Nov 20, 2020. Dr. evaluated my arm and said there could be multiple causes of the pain, including the flu shot administration. He recommended at-home exercises.  I wasn't completely satisfied with Dr. treatment response, so I scheduled 2 appointments with a chiropractor on Nov 24 and Dec 1, 2020 in an effort to treat the injury. He performed standard chiropractic adjustments, but unfortunately neither led to permanent removal of the pain.  On Jan 11, 2021 I saw Dr. again as the pain had still not dissipated. Dr. did another evaluation of my arm and this time, was more convinced I had tendinitis. He presented multiple next step options to me, but was not prescriptive with what I should do. He ultimately recommended physical therapy, but I was hesitant because my diagnosis was still not confirmed (Dr. actually advised against an MRI due to cost and complexity).  Dissatisfied with the overall approach Dr. had taken to my injury, I scheduled my first appointment with Orthopaedic on Feb 10, 2021 given their specialization. I met with a PA, who evaluated my arm and after I explained the history of my injury, stated that while rare, he believed it was likely caused by the flu shot administration. He ordered x-rays that day (which confirmed that I didn't have any type of bone injury) and prescribed the MRI for the following week so we could get a formal diagnosis. On Feb 17, 2021 I got the MRI, and on Feb 23, 2021, Dr. at Orthopaedic confirmed I had rotator cuff tendinitis. He recommended 2 weeks of Advil and physical therapy. I started physical therapy the following week and have had several PT appointments since then.  Although the pain has lessened since the very beginning, it remained over 6 months and disrupted and restricted activities. I woke up in the middle of the night several times feeling pain in the arm and needed to shift positions in order to relieve the pain and fall back asleep -- so I did not have a solid night's sleep for over 6 months. During the day, I avoided using my left arm for any weight-bearing or muscle-engaging activities, but this was near impossible as I cared for my 1 year old daughter and had to frequently lift her into her high-chair, the car seat, crib, etc. which requires using both arms and, therefore, triggered the pain. I also experienced pain when trying to lift groceries, put milk back in the fridge, open jars and bottles, pick up delivery packages from my front door, etc. Even turning the steering wheel to the left in the car triggered the pain. Again, I tried to use just the right arm but that was not always feasible or safe to do so.  I have never had a shoulder/arm injury in the past and was not engaging in any activities that could cause rotator cuff tendinitis, especially not in my left arm (as my right arm is dominant). The soreness/pain occurred immediately after the flu shot I received on Sept 20, 2020.