blood in bf baby poo but NOT cracked nipple

[toadstool]

DD2 has just had a bit of blood in her nappy (which was very mucousy, but then she's got a cold). The first time it happened was about 2 months ago over a couple of days, when I drank a lot of soya milk and ate some tofu - blood in poo. It stopped as soon as I stopped taking the soya. This time round she's taking a few solids happily for about 3 weeks, and I'm introducing her to some formula, in preparation for starting nursery this month. No probs until last night, when blood reappeared. So it's either the formula (2 oz taken, more than she usually manages, 24 hrs beforehand), or the solids (apple puree, no problems over the past month with that), or me (?random almond croissant as elevenses).
HV pretty useless, suggested she has constipation - well, I'm not a doctor, but that seems highly unlikely given her bf baby squits 4-5 times a day.
I could stop the solids as she's a tad young for them, but I suspect it's either something I'm eating, or the formula.
Any advice?

have you taken any ibuprofen lately toadstool? We had a tiny bt of blood in the nappy and in her posset when she was a few days old - the brufen had irritated her tummy.

Ah, interesting. No, I hadn't, but I did take some fizzy calcium supplement for the first time - I've been taking chewable Osteocare tablets without any ill-effects, so it could be the additives in the supplement.