I have an implant now and I haven't been to the dentist since. Recently my left cheek started swelling and my left lymph node underneath my jawline is swollen and not painful. What could be the cause?

Cheek swelling. You didnt mention if the implant is near where the swelling is. Cheek swelling could be from infection of a backed up salivary gland. The raised node is also an indication of infection. An infection in this area can become life threatening very quickly. You should seek care from a dentist or physician immediately.
Swelling/lymph node. Unfortunately, you have not provided enough information for a comprehensive diagnosis. I would suggest you see your Dentist to determine if this is even related to the implant. It could be so many other things??
See dentist ASAP. See your dentist immediately. The dentist will ascertain if this is related or not related to the implant. You do not mention if you had the tooth restored or just had the implant placed and never followed through. See your dentist to alleviate your concerns.
Swelling. There are a number of things that can cause swelling in your cheek. The only way to find out is to see a dentist for an exam, diagnosis and treatment. Don't wait any longer. Call today.
See your surgeon. for evaluation, x-rays and treatment options.
More info needed... Is implant on left side, too? Any swelling needs to immediately be addressed. Please call your dentist to have your mouth examined to determine any contributing factors.
Could. could be a number of things, not enough information from you to speculate, see a/or your dentist for better understanding of the problem/situation.
Possible infection. It sounds like there is some bacterial activity going on in that area. Get to see your dentist asap. It would be nice to save the implant, if possible.