Help me /fit/

Okay so I have started working out and losing weight thanks to you guys, which is great. Only one problem, after my last work out (only my 5th) I have quite a pain in my lower back, at the base of my spine. I have always had a weak back and the pain is not as bad as I have had in the past but it is quite painful when getting up from sitting or when bending over. I think it is a result of either my deadlifts or my military presses. What might I be doing wrong and what stretches can I use to relieve the aching? Could it just be DOMS since I rarely worked out my back before?

I also have a strain in my back just under my shoulder blade from lifting something a week or so ago (unrelated to working out). The pain doesn't seem to be clearing up and it hurts when I lift weights and even when running, which is annoying. Is this a common injury? What can I do?

I have tried to illustrate on the picture where the pain mostly is. Thanks.