Lynn Schiller has a cold. Sniffles, sore throat, headache, the whole routine. She has had it ever since she took that boat ride to Catalina Island a little while ago, underestimating the chill sea wind. To Lynn, having a cold is more than just a pain in the head, for it keeps her from doing the things she likes to do; namely, swimming, acting, dancing and singing. She is adept at all those activities, especially acting and singing. In high school and college, she sang with school bands, and during a year in Germany, she and a small combo entertained at several Army bases. She plays the guitar now and is learning piano. Her voice is high, with the lilting quality of a Joni Mitchell, and she is beginning to write her own songs--ballads, mostly, with a touch of country. When she is not singing, she takes acting lessons or ad-libs comic skits with the Ace Trucking Company, an improvisational troupe. She has just graduated from the famous Lee Strasberg Theatrical Institute and hopes to land a decent part in a movie soon. A musical would be perfect. Meantime, between dates with her boyfriend, Glenn Frey, currently of the rock group Eagles, Lynn is doing a little modeling (in Playboy's May feature "T" Formations, she is the model with the roller skates and the hose) and taking modern-dance lessons. And when she is not doing any of those, chances are you'll find her out swimming, surfing, horseback riding or playing tennis or baseball. Baseball!? "I grew up with three brothers," she says, "and learned to play all the sports little boys play." She describes herself as perky, flirtatious and occasionally aggressive ("When I know what I want, I go after it"). Generally speaking, she's extremely active; but what with this cold, she's stuck indoors, in bed. But then again, there's a whole lot a person can do indoors, in bed.