About a week ago, I was masturbating when I noticed that I suddenly lost my erection and started feeling pain whenever I tried to finish. I looked at my penis to see if everything was in order, and found a lump on the underside of the shaft! Worried, I tried again the next day, and everything was in working order again. However, today it happened again! I was trying to get it up but it just wouldn't get completely hard - the base would get hard, but anything above the lump (which, though appeared to be subsiding over the past week, was thick as ever this time) stayed fairly soft.

I've been under a lot of stress lately, so that may be a factor in all this.

I'm very worried. I'm afraid this may be Peyronie's disease but my penis doesn't appear to be bent or curved in any way. I plan to see a doctor, of course, but would like some input in the meantime: Does this seem like it's most likely Peyronie's, or could it be something else, like a lymphocele?

Thanks!

I highly doubt that the lump would cause any ED (Erectile Dysfunction).
You have not mentioned what the bump looks like?
It may be that it is a normal penile lump such as a Sebaceous Cyst; however, this does not explain the loss of the erection. It may simply be that the stress is playing a big part in this. I suggest you stop worrying but if the problem persists, consult a doctor.