what can i do to ease my back pain?

It sounds like you need to do some weight training to develop the strength in your arms. As someone else mentioned, a 7-month-old is a lot of baby. I know a kindergarten teacher who developed serious shoulder problems from lifting up the kids, so don't feel like you're alone. Your first call might be to your OB/GYN, who may have exercises specially designed for this situation. Or you can buy some weights and do some reps.

Edit: Just realized I hadn't actually answered your question about dealing with the pain now. Stretches are good, as are ice packs to help reduce inflammation of the muscles.

lower back pain or any other sort of back pain should really be looked at by a doctor, the trouble with back pain is there might be quite a few causes from muscle cramps, general aches or perhaps full blown denerative disc disease. This is why seeing a doctor is a must. If you are over 40 years old then degenerative disc disease could very well be the issue.

Well it doesn't sound like it needs surgery or you would probably be hurting all day. I'm a true believer in chiropractic care when nothing else works. It worked for me when none of the specialist had a clue.

I normally don't need to go any more since I"m not in athletics any more, but there are times I start feeling a slight discomfort in my hip which is a warning sign for me to take it easy or seek treatment.

Just bending over to pick up your baby you could have ended up with tight back muscles, they can happen so easily at times. When you are rested you are able to keep everything in alignment to keep the pain away, although I think you are using some effort you may not even be aware of to do so, but by the end of the day your whole body is so worn down that you aren't able to fight it and that allows your body to move in a way to cause more pressure on the nerves to cause the pain. Once rested up you can fight through most of the day again but who likes to fight through them? To get rid of back pains you have to free up your back muscles to release the ones that are causing the pains and here's how to free them up:

Back:

(do while sitting on a chair)

Place your left hand on your left leg next to your body. Place your right hand over your left shoulder, fingers over the back and the palm in the front, and firmly pull down on them and hold. After 30 seconds slowly lower your body forward and to the outside of your left leg, keeping your left arm as straight as possible. When you reach your lap remain there for another 10 seconds, then release the pressure but rest there for another 30 seconds. Then reverse your hand positions and do your right side.

For best results relax your body first by taking a deep breath and exhaling then remain this relaxed.

Posture? A 7 month old is a lot of baby. Use warm compresses and yoga. Hot tea may help you relax and let go of stress.