Hard to say without doing an assessment. Could be a muscle strain or a nerve impingement. If pain or discomfort persists at rest for more than a week or two, Id suggest going to see a PT or Orthopedic Specialist. Better to address it immediately, than have it persist for weeks into your training schedule.

Your Sciatic nerve is the largest nerve in your body, running from the lower vertebrae to the feet, and prone to compression at several spots. Signs and symptoms vary, ranging from sharp to dull pain in the lower back to burning, numbness, or tingling down the legs and into the foot. If the compression originates in your lumbar vertebrae, you ll just have to wait for the irritation to cool off. Treatment involves rest, followed by glute, hamstring, and core strengthening exercises. I suggest trying some glute bridges, front and side planks if pain free. Begin with 3 sets of 15 for the bridges, and 3 sets of :20-:60 for the planks, until symptom free and then progress back into your lower body strength training exercises.

Sometimes, the Piriformis, a little muscle underneath the glute, becomes tight and or overactive, and can compress the nerve. Foam rolling your lower body, and adding these stretches may also help alleviate symptoms. Running in the absence of strength training, can create instability and weakness in the core, hips, and legs. Its a repetitive, quad dominant activity that can lead to muscular imbalances, if not balanced out with strength training, especially the posterior chain, and soft tissue work with a foam roller, and stretching. Give it a rest and then try the following foam rolling and strength training exercises several times per week, especially before runs, and the stretches several days a week, especially after runs.

Foam Roll each area for :30-:60 seconds, focusing on tender spots. Follow immediately with glute and core activation work. Stop if symptoms persist.

posterior chain tightness exercises — IT Band

Glutes

Glute Activation - 3 sets of 10-15 repetitions

posterior chain pain - glute bridge — Glute Bridge Start

Glute Bridge Finish

Lateral Band Walk - Glute Med

Core Activation - Hold for 3 sets of :20-:60 seconds

Front Plank posterior chain stretches - exercises for reader — Front Plank

Side Plank

Pirifomis stretch - Lying supine on the floor, grab your right knee with left hand, and gently pull across your midline and towards your opposite shoulder, until a mild stretch is outer hip/glute. Breathe and hold for :30-:60 seconds, 2-4 sets on both sides. Stop if symptoms are present.

Glute/external rotator stretch - Bend your left knee and place your left foot on the floor, cross your right foot over your left knee. Reach through and grab the back on your left thigh, and gently pull your leg into your chest until you feel a light stretch. Relax your head and shoulders while you take a few deep breaths for a count of :30-:60 seconds, 2-4 sets on both sides. Stop if symptoms are present.

external rotator srtetch — Glutes/External Rotator Stretch

For more information please see my pages on foam rolling, glute and core activation

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Reader Question - What should I do about posterior leg pain during/after running?