"Why Am I So Sore?"
The basic principle of lifting weights is that when you exercise the muscles, little micro tears occur, then they repair, and thats how you get stronger.
Soreness is a result of damaging the muscles (in a good way) and your body healing.
The amount of soreness can vary depending on a couple things:
New Stimulus
If a muscle hasn't been exercised in a while (or ever), or if a new stimulus is introduced (you're used to rep ranges 3-5, but you do 12-15 reps), this will cause more muscle breakdown, and a greater response (soreness)
Deyhydration
Staying hydrated is a great way to prevent excessive soreness
Poor recovery
If your body has other things to worry about (ie: poor sleep, poor nutrition, drinking alcohol, being mentally stressed out,, etc) it will prioritze those things before taking care of soreness. Muscle repair is not high on the list of priorities to live, so it may take longer to recover if other areas are out of sorts
What can I do to get "Un-Sore"?
Move
Moving and contracting muscles is another great stimulus to tell the body "hey, we need to use these things now, so you better get your s@#$ together
Hydrate
Better hydration = higher blood viscosity = easier removal of waste
Glutamine
This is one of my favoirte suppliments! It's main function is gut health, but as a consequence, it helps with mild soreness.
"How do I know if I'm injured or sore?"
The easiest way to answer this is start to move. If the sore area does not feel any better after a solid 10 min warmup, it may be best to avoid that area, or take a rest day.
In my 10+ years of experience, its about a 200:1 that people say "I'm glad I got moving" vs "I should have stayed home".