I was travelling on the Northbound Jubilee Line, being watched. At first it was furtive, surreptitious glances, out of the corner of her eyes. The sort of glances that are pretty commonplace on public transport. But they soon progressed to an unabashed stare and eventually she was virtually craning her neck to look at me.
I peered down at myself, to check that I hadn't accidentally left the house wearing my breakfast down my front or, worse, wearing my underwear Superman style. I wasn't.
Less than a minute passed. She turned to the girl travelling with her. "What is that?" she said, quite audibly.
This wasn't a curious 'Well isn't that strange, I wonder what it could be' enquiry. The edge to her voice betrayed that she wasn't asking her companion a question; she was confiding her fear.
The travel companion's eyes flashed at me with a slight panic that was also evident in her voice as she said "What, that thing under her clothes?"
A couple of other heads in the carriage whipped round to look.
I knew what they were thinking.
The train rattled noisily into my station. I diffused their panic by slipping off.
When I got home I stood in front of the mirror. I saw my pump in my pocket, a loop of tubing going to my tubeguard and tubing snaking from there up under my clothes. On my arm the Guardian sensor was just visible, its grey wire feeding up under my sleeve. The outline of the transmitter on the back of my shoulder was clear through my shirt. The Guardian monitor was clipped into the mesh pocket on the side of my backpack. I reached into my backpack to look at it...
It doesn't really surprise me that people were suspicious. The terror threat has been dragged brutally back to the forefront of everyone's minds - not that it was ever far away - by today's foiled terror plot, the banning of aircraft hand baggage and ensuing airport chaos and the raising of the UK threat level to critical.
And I clearly remember how just over a year ago, after the 7/7 bomb attacks and subsequent Stockwell shooting, a friend told me, with genuine concern, to make sure I kept my pump well concealed and most importantly without the tubing hanging out while on public transport.
Because, I guess, these days everyone's a suspect.