Lower Back Pain Only Day After Exercising

Good day,
  I weight train 3 times a week and am very careful in how I train and how I lift the weights. I avoid exercises that could compress my spine. Lately though regardless of what exercise I do, I have lower back pain as the next day progresses.

Note that I can lift as heavy as I want during my workout with no pain what so ever and even the following day if I was to have pain and lifted again, I would not feel pain.

This is what's baffling me because I would think I would feel pain during weight training if anything but I never do. Just the next day and sitting is usually the issue.

I was therefore wondering if it's tight muscles, inflamed muscles, or so on?

I've gone to doctors in the past and I was told I had a herniated disc. Ok, but if that was the cause, how would I be able to lift heavy weights completely pain free every time?

If I stop working out for a week then I have no pain sitting or anything. As soon as I start to lift weights again, it comes back the day after. Sometimes I feel it's all in my head.

Any suggestions would be appreciated.

Thank you in advance for all responses.

Jay