Red lump on puppy fore arm?

I noticed a spot on my 15 month old puppies arm 2 days ago it had scabbed over, I bathed it with puppy antiseptic and it burst. I noticed today its doubled in size and turned hard. the vets is not open til morning. ive googled it and google showed similar images indicating it to be a tumor. is it just a spot or could it be a tumor? please help. dont want to be melodramatic about a spot or am I right to worry??? thanks in advance