My head-canon for Sam's medical expertise is that following early episode - erm mission - whumpage, and Daniel having to play midwife, Jack ordered Sam to take some training courses so that they'd have a 'mini-medic' on team. Being the super-bright scholar that she is, Sam wouldn't be content with superficial knowledge. She'd bone up on each topic until she felt confident she had it sewn up [puns fully intended]