It is not necessarly a bad sign. What you might want to do for the interem is to only lift weights on the machine not free weights and also back down your work out to a point that you are not in too much pain.
After a few weeks of that work out, step it up very slowly, if you have true back problems, nothing will stop them but you can build up your tolerance to pain stimulation by moving very slowly as far as the intensity of your work out.
It is always a good idea to go to an orthopedic or neuro doc when dealing with back pain, don't be surprised if your herniations are not any different than they were in the first mri, that is the way it goes. Having a herniation does not mean that you cannot work out. If you are not experiencing any pain or minimal pain then you are ok. The spinal cord normally ends at l1/l2 or maybe l2/l3 and since your injuries are below those levels, you cannot hirt your spinal cord and so you should be ok.
Remember, l4/l5 is where the siatic nerve roots are and so if there are major/minor herneations pressing on the nerve roots, there is a good chance you are going to experience problems otherwise if your bulges are away from the nerve tissue and if your pain is muscle spasm related vs. Spinal pain you are likly to be refered back to pt and told to keep up the workouts so that you can build up your back strength.
Good luck,
brian