I had a low lying placenta with my twin pregnancy and bled on and off from 23 weeks onwards. Some episodes triggered labour (the bleeding can irritate the uterus, trigger contractions and therefore start labour) so they tend to put you on something called 'pelvic rest' which essentially means nothing that could jar the cervix or irritate the uterus (so no sex, orgasms, exercise, overdoing it). Boring but safe.
I found it very scary - the bleeds came with no warning and varied from the light (a minute of red blood) to pouring out of me. I was in hospital from 30 weeks on bed rest and eventually had an emergency caesarean at 35 weeks after a massive bleed.
The bleeding is common with placenta praevia and although there is no guarantee that it will happen again, there is also no guarantee that it won't. If you've had a bleed, it's more likely to happen again than not. However, the placenta can move out of the way (as the uterus grows upwards, the placenta gets 'stretched' away from the cervix, which removes the risk). Mine stretched away a bit but was unstable at one edge (they think a bit had 'peeled' away, which is why I was very unlucky and the bleeds persisted.
Try to take things easy in the meantime. They'll have a much clearer idea once you've been scanned and the bonus is that you'll now have a lot more scans and probably consultant care as you'll be slightly higher risk. It's very scary and I really sympathise, but it doesn't spell doom - my twins are now happy, healthy two year olds and weren't remotely affected by all the dramas of my pregnancy.
Good luck and hope things settle down - there is every chance that this will be a one off.
Kx