Knot found on my dog

Question
Hi There,

Thank you in advance for offering your services...
I have a five year-old male Maltese (Chancey) who I noticed, as of this morning, is acting differently.  Normally, when I first wake up, Chancey is very hyper and exciteable, but this morning, he was very subdued and timid.  I thought that perhaps he had had an accident in the house and was "sheepish" because of it, but that wasn't the case.  Normally, when I ask him to "come" he does so happily, but this morning he was very slow to respond.  He also seemed to be shivering.  I will say, that Chancey is very sensitive, so I even thought that maybe one of my roommates had yelled at him to stop doing something (not that they're known to do that) or maybe another dog scared him at the dog park when I wasn't looking and he's just traumatized.  
Before leaving for work this morning I tried looking him over and touching to see if he was in pain anywhere or had any marks on him; I didn't see any.  We have seen some black widows around our yard, so I thought that maybe he could have gotten bit.  My boyfriend took Chancey to his house today so he could keep an eye on him.  He just called me and said Chancey was still acting weird and that he found a knot on Chancey's leg that seemed to be hard and Chancey flinched and tried to bite him when my boyfriend tried to touch it, which Chancey is only known to do when something hurts him.  Do you have any idea what this could be or what dangerous symptoms I should be looking for?  Is this likely something that will go away on its own (like a bug bite)?  I'm trying to determine if I should leave work to take him to the vet.

Answer
Brooke,

You should absolutely take your dog to a veterinarian as soon as possible.

Any time a dog seems suddenly disoriented, you have a red flag. This, combined with the knot is reason enough for concern. Please see Chancey's doctor as soon as you can.

Thanks for your question and good luck. I'm sure Chancey will be fine if he receives treatment quickly. Let me know what you find out.

Ashley