In a word, yes; depending on the individual case, fan art can indeed infringe copyright.

Copyright is a bundle of EXCLUSIVE rights held by the copyright's owner. Among those rights is the EXCLUSIVE right to create and distribute "derivative works," which most fan art qualifies as.

The website where you posted your fan art clearly takes its responsibilities under the Digital Millennium Copyright Act (DMCA) seriously. It may be that the site received a complaint under DMCA about your materials (in which case they have a limited time in which to remove the offending material), or it may be that they found the material by themselves and acted proactively to remove the potentially infringing materials.

Removing potentially infringing materials without notice is absolutely fine under DMCA. The ISP is under no obligation to show you a copy of the notice; in fact, it's good business policy for an ISP to keep those notices under wraps. Illegal seizure is not an issue in civil matters such as this one; only law enforcement officers (police, FBI, CIA, etc.) can illegally seize evidence in a criminal matter.

You can hire an attorney to help you sue the ISP in federal court to ask the judge to rule that your work does not infringe the copyright in question. That is a long, expensive endeavor, in which neither you nor the ISP is guaranteed a successful outcome.

My best tip? Find your own artistic expression.

THIS POST CONTAINS GENERAL INFORMATION AND IS INTENDED FOR ENTERTAINMENT PURPOSES ONLY. IT DOES NOT CONSTITUTE LEGAL ADVICE, NOR DOES IT CREATE ANY ATTORNEY-CLIENT RELATIONSHIP. FOR LEGAL ADVICE ON YOUR PARTICULAR MATTER, CONSULT YOUR ATTORNEY.