I'm a fit, healthy, 26 year-old male. over the past few weeks i have noticed a drop in libido, and it's becoming problematic, what could be the cause?

Many causes: There are many possibilities: sleep deprivation, depression, anxiety, relationship conflict, missing a former lover, life stresses, substance abuse, a medication side effect, or a medical problem are some causes. If your libido doesn't return or if you are experiencing any of the above, see your primary care doctor for advice and/or direction. Hope this helps.