What Causes Spasms In The Lower Back?

HI- I threw out my back when lifting a box over weekend- Saturday. It is sore, but has not impacted my routines/work. Occasionally - after sitting at desk or getting up from bed I will have a very brief spasm in my lower back. I also find if I squat or stretch the discomfort subsides. Sometimes during the day I don t even notice it. other times my lower back feels tight. No numbness, no pain in legs. I can walk fine, just sometimes back feels tight. I take an advil 3 or 4 times a day and have iced my back a handful of times. Should I now start heat? Are there certain stretches I should do? How long is the discomfort likely to last. This is the first time this has happened. I am pretty active. I travel often- mostly air travel and also some driving. If I stand up while on my computer instead of sitting on desk chair I do not seem to tighten up. No spasms except this AM when I got out of bed. Any suggestions? I travel for work and have an international trip next week. I also have an all day presentation to a client and will be standing all day on Wed. I need to try to accelerate healing. Have not contacted my doc yet. Thank you Carol