Painful lump on top of foot

Noticed a bit of mild discomfort on the tops of both feet during a long offroad run four days ago. Mostly when running downhill, and just underneath where the bow in my laces was tied. Loosened off my laces a bit and carried on, and it felt okay.

Couple more runs, trail and tarmac, no real bother. Mild discomfort and I assumed I'd just tied my laces too tight, and slackened them off mid-run.

Till last night when I was road running and I had to loosen off my left lace three times during the run. Once loosened right off I could run no problem. When I got home and had a closer look, I found quite a lump (I think) on the tendon that attaches to my big toe. Round about where the top of my laces sit, toward the inside of my foot.

Does anyone have any ideas what this is and how to deal with it? Its not sore in the slightest to walk around barefoot or in slip-on shoes, but laces are painful. I'll be calling my physio first thing tomorrow, but just wondering if anyone could help before then...

Shoes and socks are same ones I've been wearing for a while. Brooks/Asics neutral cushioned for road, Adidas Kanadia for offroad. Weekly mileage is roughly 45-50-60-35 on a gradually building 4 week cycle.

I'd be interested to hear what your physio said. I had a similar problem last summer. Was never sure what caused it, decided possibly strain on ligaments at the tarso-metatarsal joint, or something like that. To run, I used a piece of chiropodist's felt, cutting a hole in the middle (to take all pressure off the lump, then attaching it to the foot using zinc oxide tape wrapped all the way around the foot (in case I needed the support). I used this for several weeks (same piece of felt can be used several times so long as you attach it with tape rather than use its own sticky backing) and it worked for me. It no longer hurts, but I still (eight or nine months later) have a slight lump.