A constitutionalist, also called an originalist, is a person who advocates in political and governmental adhrerence to the The Constitution of the United States of America,
as written and intended by the original drafters. Jon Roland, self-proclaimed "prominent constitutionalist," notes that this governmental philosophy also incorporates adherence to the Anglo-American common law which does not conflict with the Constitution.
U.S. Supreme Court Justice Antonin Scalia is considered one of the current leaders of constitutionalist thought in the United States.
A libertarian, by contrast, believes in a small, limited government. A libertarian government would give great deference to individual rights and limited government action at all levels.
To a libertarian, a constitution is a device to keep the government's powers in check with respect to individuals. A libertarian, for example, would oppose a constitutional amendment to prohibit flag desecration, on the grounds that such an amendment would expand the government's power. A constitutionalist may support such an amendment, so long as it were properly ratified under the procedures set forth in the original document.
A constituionalist would likely disagree that the the U.S. Constitution contains a right to privacy, as the Supreme Court held in Roe v. Wade in 1971. A libertarian, however, might support the notion on the grounds that an individual right to privacy further limited federal government to regulate individual behavior.
So if you consider yourself a libertarian and a constitutionalist, you're probably one or the other, even though there may be significant overlap between the two.