Mike nailed the problem - a light leak.
If you are going to keep the K1000 I would suggest that you replace
all of the light seals on the camera. It is not a daunting task at all; in fact, it can be pleasurable and definitely rewarding. It brings you a little closer to the camera and, in my case, I gained a new found respect for the care and attention that went into the original design. When I acquire an older film camera I always replace the light seals. A great resource for this task is
Jon Goodman. Most SLR's are pretty straightforward in design and placement of light seals but a quick email to Jon asking about specifics could be beneficial.
I purchased one of his large 'kits' a couple years back when I was determined to bring a Canon GIII QL back to life. A trip to a local hobby store got me an X-acto knife and blades, a fine-point set of tweasers, and a decent metal straight edge; necessary tools to do a proper job. The little Canon was probably
not an easy first camera to replace the light seals on as the removal of the back door and pressure plate proved to be challenging. The K1000 should be a snap to work on, though.
My point is, if you are going to replace the felt on the hinge side why not take the extra few minutes and do all the seals including the mirror damper. My ME Super, which is newer than your K1000, had a severely messed up mirror damper; a little care has brought it back to new condition.
These old cameras are great, and meant to last many years, but only with proper care and maintenance.
Good luck!