RichardB, it is a fact that smoking marijuana increases your heart rate, which gives you the feeling of being scared, nervous or anxious... hence paranoia.

"Certainly the least pleasant mental effect of cannabis, though it is one you can learn to counter. Partly it comes about through the increase in heart rate, which can cause a feeling of slight panic and dread if you are not used to the sensation. You will be more likely to jump at loud noises or unexpected bodily contact. Busy places can become difficult and confusing (like Amsterdam Central Station, for one). You may feel as if you are being followed or watched. In extreme cases you can really freak out over this: literally everyone will be watching you, and if asked, you can prove it. I am just trying to be as honest as possible. Paranoia is unpleasant, but is just a temporary thing and can be combatted. If affected too badly by it, just remind yourself that it is an effect of the drug and like all the others, will wear off over time. If possible, get yourself somewhere quiet to chill out; solitude is a good way to combat it. Or find something to concentrate on, like a calming or interesting programme on TV, or a piece of favourite music. It's usually better to acknowledge you are a bit freaked out than to try and brazen it out in front of others, which can often only make you more paranoid."

http://homepages.poptel.org.uk/DrDrew/health.html