Here's the thread from the other day about the same incident:
http://www.medhelp.org/posts/Anxiety/should-i-freak-out/show/1797581#post_8276544
Several of us have told you there is NOTHING to worry about. Your doctor would have reacted if it was a concern. Pulse oximetry is not a perfect science, and especially being pregnant, it will vary. Anxiety will also affect the levels.
Greenlydia is right, do NOT buy a home pulse ox machine. You will become obsessed, and every reading that is slightly lower than you would like to see will spiral you right into anxiety.
Put this behind you, you seriously have nothing to worry about.