Your entire spine, from your head, all the way down to your tailbone consists of many bony structures called 'vertebrae' sandwiched alternatively between gel-like structures named 'discs'. Often, whilst doing something relatively innocuous such as picking up a towel off the ground, or whilst doing something beyond your capacity, like lifting a small 3-door car above your head, you may cause a sudden 'disc bulge'.
A disc bulge refers to a displacement or movement of the gel-like disc beyond its usual position. When a disc shifts position, it often causes a significant lack of movement and pain. The disc may also compress structures nearby such as a lumbar nerve, which can cause referral of pain travelling down typically one of your legs. This is what people often refer to as 'Sciatica', which gets its name from the sciatic nerve which runs from your spine, through your buttock and down your leg.
The good news. A very high percentage of disc bulge injuries will completely resolve with the assistance of a qualified Physiotherapist. However, this may take weeks to fully recover.