Nathan, I'll say the reassuring part first, and then the part that's actually useful.
A typical adult platelet count runs roughly 150 to 400, and a result just below the lower line on a single test, with everything else on the blood count normal and no symptoms, is common and very often not clinically significant. Reference ranges are built so that a few percent of healthy people fall outside them on any given day. Counts also move with recent viral infections, some medicines, alcohol, and lab factors such as platelets clumping in the tube, which can make the count read low when it isn't. That is exactly why the standard response is the one your GP gave: repeat it in a few weeks and look at the trend.
What a clinician weighs is the whole picture rather than one line. How far below the range, whether other cell lines on the count are also abnormal, whether there's unusual bleeding or bruising, and whether the number goes up, holds, or falls on repeat. The frightening causes you read about usually look quite different from a single mild dip in someone who feels well. I can't tell you what yours is, only that the plan you've been given is the normal, sensible one.
Practical bits for the repeat: tell them about the cold, mention any medicines or supplements, and if you notice anything new in the meantime, such as bruising you can't explain, nosebleeds or bleeding gums that won't stop, or tiny red spots on the skin, don't wait for the appointment. The full blood count guide explains each section of the report, and understanding your blood test results covers why a flagged value on its own says less than it looks.
