No, this is quite common in older houses. The explanation is here.
http://www.diynot.com/forums/viewtopic.php?t=4 4684
The forces work as an equilateral triangle, with the base of the triangle being the line of bricks just above the frame, rising to an apex centrally above the frame (about 700mm vertically above it on a 900mm width door frame). So the force exerted on the frame is only a maximum of the weight of the bricks inside this 'triangle', and it is only ever these that could in theory fall out - not the whole stack of masonry above.
You should be able to solve this without installing a whole new concrete lintel, which would require cutting away at the masonry at the sides to give the lintel suitable bearing points. Go for the galvanised angle option, as discussed in the weblink.